api, login, auth

1 files changed, 4056 insertions, 0 deletions

diff --git a/node_modules/mongoose/lib/document.js b/node_modules/mongoose/lib/document.js
new file mode 100644
index 0000000..9e8cb9a
--- /dev/null
+++ b/node_modules/mongoose/lib/document.js
@@ -0,0 +1,4056 @@
+'use strict';
+
+/*!
+ * Module dependencies.
+ */
+
+const EventEmitter = require('events').EventEmitter;
+const InternalCache = require('./internal');
+const MongooseError = require('./error/index');
+const MixedSchema = require('./schema/mixed');
+const ObjectExpectedError = require('./error/objectExpected');
+const ObjectParameterError = require('./error/objectParameter');
+const ParallelValidateError = require('./error/parallelValidate');
+const Schema = require('./schema');
+const StrictModeError = require('./error/strict');
+const ValidationError = require('./error/validation');
+const ValidatorError = require('./error/validator');
+const VirtualType = require('./virtualtype');
+const promiseOrCallback = require('./helpers/promiseOrCallback');
+const cleanModifiedSubpaths = require('./helpers/document/cleanModifiedSubpaths');
+const compile = require('./helpers/document/compile').compile;
+const defineKey = require('./helpers/document/compile').defineKey;
+const flatten = require('./helpers/common').flatten;
+const get = require('./helpers/get');
+const getEmbeddedDiscriminatorPath = require('./helpers/document/getEmbeddedDiscriminatorPath');
+const handleSpreadDoc = require('./helpers/document/handleSpreadDoc');
+const idGetter = require('./plugins/idGetter');
+const isDefiningProjection = require('./helpers/projection/isDefiningProjection');
+const isExclusive = require('./helpers/projection/isExclusive');
+const inspect = require('util').inspect;
+const internalToObjectOptions = require('./options').internalToObjectOptions;
+const mpath = require('mpath');
+const queryhelpers = require('./queryhelpers');
+const utils = require('./utils');
+const isPromise = require('./helpers/isPromise');
+
+const clone = utils.clone;
+const deepEqual = utils.deepEqual;
+const isMongooseObject = utils.isMongooseObject;
+
+const arrayAtomicsBackupSymbol = Symbol('mongoose.Array#atomicsBackup');
+const arrayAtomicsSymbol = require('./helpers/symbols').arrayAtomicsSymbol;
+const documentArrayParent = require('./helpers/symbols').documentArrayParent;
+const documentIsSelected = require('./helpers/symbols').documentIsSelected;
+const documentIsModified = require('./helpers/symbols').documentIsModified;
+const documentModifiedPaths = require('./helpers/symbols').documentModifiedPaths;
+const documentSchemaSymbol = require('./helpers/symbols').documentSchemaSymbol;
+const getSymbol = require('./helpers/symbols').getSymbol;
+const populateModelSymbol = require('./helpers/symbols').populateModelSymbol;
+const scopeSymbol = require('./helpers/symbols').scopeSymbol;
+
+let DocumentArray;
+let MongooseArray;
+let Embedded;
+
+const specialProperties = utils.specialProperties;
+
+/**
+ * The core Mongoose document constructor. You should not call this directly,
+ * the Mongoose [Model constructor](./api.html#Model) calls this for you.
+ *
+ * @param {Object} obj the values to set
+ * @param {Object} [fields] optional object containing the fields which were selected in the query returning this document and any populated paths data
+ * @param {Object} [options] various configuration options for the document
+ * @param {Boolean} [options.defaults=true] if `false`, skip applying default values to this document.
+ * @inherits NodeJS EventEmitter http://nodejs.org/api/events.html#events_class_events_eventemitter
+ * @event `init`: Emitted on a document after it has been retrieved from the db and fully hydrated by Mongoose.
+ * @event `save`: Emitted when the document is successfully saved
+ * @api private
+ */
+
+function Document(obj, fields, skipId, options) {
+  if (typeof skipId === 'object' && skipId != null) {
+    options = skipId;
+    skipId = options.skipId;
+  }
+  options = Object.assign({}, options);
+  const defaults = get(options, 'defaults', true);
+  options.defaults = defaults;
+
+  // Support `browserDocument.js` syntax
+  if (this.schema == null) {
+    const _schema = utils.isObject(fields) && !fields.instanceOfSchema ?
+      new Schema(fields) :
+      fields;
+    this.$__setSchema(_schema);
+    fields = skipId;
+    skipId = options;
+    options = arguments[4] || {};
+  }
+
+  this.$__ = new InternalCache;
+  this.$__.emitter = new EventEmitter();
+  this.isNew = 'isNew' in options ? options.isNew : true;
+  this.errors = undefined;
+  this.$__.$options = options || {};
+  this.$locals = {};
+  this.$op = null;
+
+  if (obj != null && typeof obj !== 'object') {
+    throw new ObjectParameterError(obj, 'obj', 'Document');
+  }
+
+  const schema = this.schema;
+
+  if (typeof fields === 'boolean' || fields === 'throw') {
+    this.$__.strictMode = fields;
+    fields = undefined;
+  } else {
+    this.$__.strictMode = schema.options.strict;
+    this.$__.selected = fields;
+  }
+
+  const requiredPaths = schema.requiredPaths(true);
+  for (const path of requiredPaths) {
+    this.$__.activePaths.require(path);
+  }
+
+  this.$__.emitter.setMaxListeners(0);
+
+  let exclude = null;
+
+  // determine if this doc is a result of a query with
+  // excluded fields
+  if (utils.isPOJO(fields)) {
+    exclude = isExclusive(fields);
+  }
+
+  const hasIncludedChildren = exclude === false && fields ?
+    $__hasIncludedChildren(fields) :
+    {};
+
+  if (this._doc == null) {
+    this.$__buildDoc(obj, fields, skipId, exclude, hasIncludedChildren, false);
+
+    // By default, defaults get applied **before** setting initial values
+    // Re: gh-6155
+    if (defaults) {
+      $__applyDefaults(this, fields, skipId, exclude, hasIncludedChildren, true, {
+        isNew: this.isNew
+      });
+    }
+  }
+
+  if (obj) {
+    // Skip set hooks
+    if (this.$__original_set) {
+      this.$__original_set(obj, undefined, true);
+    } else {
+      this.$set(obj, undefined, true);
+    }
+
+    if (obj instanceof Document) {
+      this.isNew = obj.isNew;
+    }
+  }
+
+  // Function defaults get applied **after** setting initial values so they
+  // see the full doc rather than an empty one, unless they opt out.
+  // Re: gh-3781, gh-6155
+  if (options.willInit && defaults) {
+    EventEmitter.prototype.once.call(this, 'init', () => {
+      $__applyDefaults(this, fields, skipId, exclude, hasIncludedChildren, false, options.skipDefaults, {
+        isNew: this.isNew
+      });
+    });
+  } else if (defaults) {
+    $__applyDefaults(this, fields, skipId, exclude, hasIncludedChildren, false, options.skipDefaults, {
+      isNew: this.isNew
+    });
+  }
+
+  this.$__._id = this._id;
+
+  if (!this.$__.strictMode && obj) {
+    const _this = this;
+    const keys = Object.keys(this._doc);
+
+    keys.forEach(function(key) {
+      if (!(key in schema.tree)) {
+        defineKey(key, null, _this);
+      }
+    });
+  }
+
+  applyQueue(this);
+}
+
+/*!
+ * Document exposes the NodeJS event emitter API, so you can use
+ * `on`, `once`, etc.
+ */
+utils.each(
+  ['on', 'once', 'emit', 'listeners', 'removeListener', 'setMaxListeners',
+    'removeAllListeners', 'addListener'],
+  function(emitterFn) {
+    Document.prototype[emitterFn] = function() {
+      return this.$__.emitter[emitterFn].apply(this.$__.emitter, arguments);
+    };
+  });
+
+Document.prototype.constructor = Document;
+
+for (const i in EventEmitter.prototype) {
+  Document[i] = EventEmitter.prototype[i];
+}
+
+/**
+ * The documents schema.
+ *
+ * @api public
+ * @property schema
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.schema;
+
+/**
+ * Empty object that you can use for storing properties on the document. This
+ * is handy for passing data to middleware without conflicting with Mongoose
+ * internals.
+ *
+ * ####Example:
+ *
+ *     schema.pre('save', function() {
+ *       // Mongoose will set `isNew` to `false` if `save()` succeeds
+ *       this.$locals.wasNew = this.isNew;
+ *     });
+ *
+ *     schema.post('save', function() {
+ *       // Prints true if `isNew` was set before `save()`
+ *       console.log(this.$locals.wasNew);
+ *     });
+ *
+ * @api public
+ * @property $locals
+ * @memberOf Document
+ * @instance
+ */
+
+Object.defineProperty(Document.prototype, '$locals', {
+  configurable: false,
+  enumerable: false,
+  writable: true
+});
+
+/**
+ * Boolean flag specifying if the document is new.
+ *
+ * @api public
+ * @property isNew
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.isNew;
+
+/**
+ * The string version of this documents _id.
+ *
+ * ####Note:
+ *
+ * This getter exists on all documents by default. The getter can be disabled by setting the `id` [option](/docs/guide.html#id) of its `Schema` to false at construction time.
+ *
+ *     new Schema({ name: String }, { id: false });
+ *
+ * @api public
+ * @see Schema options /docs/guide.html#options
+ * @property id
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.id;
+
+/**
+ * Hash containing current validation errors.
+ *
+ * @api public
+ * @property errors
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.errors;
+
+/**
+ * A string containing the current operation that Mongoose is executing
+ * on this document. May be `null`, `'save'`, `'validate'`, or `'remove'`.
+ *
+ * ####Example:
+ *
+ *     const doc = new Model({ name: 'test' });
+ *     doc.$op; // null
+ *
+ *     const promise = doc.save();
+ *     doc.$op; // 'save'
+ *
+ *     await promise;
+ *     doc.$op; // null
+ *
+ * @api public
+ * @property $op
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$op;
+
+/*!
+ * ignore
+ */
+
+function $__hasIncludedChildren(fields) {
+  const hasIncludedChildren = {};
+  const keys = Object.keys(fields);
+
+  for (const key of keys) {
+    const parts = key.split('.');
+    const c = [];
+
+    for (const part of parts) {
+      c.push(part);
+      hasIncludedChildren[c.join('.')] = 1;
+    }
+  }
+
+  return hasIncludedChildren;
+}
+
+/*!
+ * ignore
+ */
+
+function $__applyDefaults(doc, fields, skipId, exclude, hasIncludedChildren, isBeforeSetters, pathsToSkip) {
+  const paths = Object.keys(doc.schema.paths);
+  const plen = paths.length;
+
+  for (let i = 0; i < plen; ++i) {
+    let def;
+    let curPath = '';
+    const p = paths[i];
+
+    if (p === '_id' && skipId) {
+      continue;
+    }
+
+    const type = doc.schema.paths[p];
+    const path = p.indexOf('.') === -1 ? [p] : p.split('.');
+    const len = path.length;
+    let included = false;
+    let doc_ = doc._doc;
+
+    for (let j = 0; j < len; ++j) {
+      if (doc_ == null) {
+        break;
+      }
+
+      const piece = path[j];
+      curPath += (!curPath.length ? '' : '.') + piece;
+
+      if (exclude === true) {
+        if (curPath in fields) {
+          break;
+        }
+      } else if (exclude === false && fields && !included) {
+        if (curPath in fields) {
+          included = true;
+        } else if (!hasIncludedChildren[curPath]) {
+          break;
+        }
+      }
+
+      if (j === len - 1) {
+        if (doc_[piece] !== void 0) {
+          break;
+        }
+
+        if (typeof type.defaultValue === 'function') {
+          if (!type.defaultValue.$runBeforeSetters && isBeforeSetters) {
+            break;
+          }
+          if (type.defaultValue.$runBeforeSetters && !isBeforeSetters) {
+            break;
+          }
+        } else if (!isBeforeSetters) {
+          // Non-function defaults should always run **before** setters
+          continue;
+        }
+
+        if (pathsToSkip && pathsToSkip[curPath]) {
+          break;
+        }
+
+        if (fields && exclude !== null) {
+          if (exclude === true) {
+            // apply defaults to all non-excluded fields
+            if (p in fields) {
+              continue;
+            }
+
+            try {
+              def = type.getDefault(doc, false);
+            } catch (err) {
+              doc.invalidate(p, err);
+              break;
+            }
+
+            if (typeof def !== 'undefined') {
+              doc_[piece] = def;
+              doc.$__.activePaths.default(p);
+            }
+          } else if (included) {
+            // selected field
+            try {
+              def = type.getDefault(doc, false);
+            } catch (err) {
+              doc.invalidate(p, err);
+              break;
+            }
+
+            if (typeof def !== 'undefined') {
+              doc_[piece] = def;
+              doc.$__.activePaths.default(p);
+            }
+          }
+        } else {
+          try {
+            def = type.getDefault(doc, false);
+          } catch (err) {
+            doc.invalidate(p, err);
+            break;
+          }
+
+          if (typeof def !== 'undefined') {
+            doc_[piece] = def;
+            doc.$__.activePaths.default(p);
+          }
+        }
+      } else {
+        doc_ = doc_[piece];
+      }
+    }
+  }
+}
+
+/**
+ * Builds the default doc structure
+ *
+ * @param {Object} obj
+ * @param {Object} [fields]
+ * @param {Boolean} [skipId]
+ * @api private
+ * @method $__buildDoc
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__buildDoc = function(obj, fields, skipId, exclude, hasIncludedChildren) {
+  const doc = {};
+
+  const paths = Object.keys(this.schema.paths).
+    // Don't build up any paths that are underneath a map, we don't know
+    // what the keys will be
+    filter(p => !p.includes('$*'));
+  const plen = paths.length;
+  let ii = 0;
+
+  for (; ii < plen; ++ii) {
+    const p = paths[ii];
+
+    if (p === '_id') {
+      if (skipId) {
+        continue;
+      }
+      if (obj && '_id' in obj) {
+        continue;
+      }
+    }
+
+    const path = p.split('.');
+    const len = path.length;
+    const last = len - 1;
+    let curPath = '';
+    let doc_ = doc;
+    let included = false;
+
+    for (let i = 0; i < len; ++i) {
+      const piece = path[i];
+
+      curPath += (!curPath.length ? '' : '.') + piece;
+
+      // support excluding intermediary levels
+      if (exclude === true) {
+        if (curPath in fields) {
+          break;
+        }
+      } else if (exclude === false && fields && !included) {
+        if (curPath in fields) {
+          included = true;
+        } else if (!hasIncludedChildren[curPath]) {
+          break;
+        }
+      }
+
+      if (i < last) {
+        doc_ = doc_[piece] || (doc_[piece] = {});
+      }
+    }
+  }
+
+  this._doc = doc;
+};
+
+/*!
+ * Converts to POJO when you use the document for querying
+ */
+
+Document.prototype.toBSON = function() {
+  return this.toObject(internalToObjectOptions);
+};
+
+/**
+ * Initializes the document without setters or marking anything modified.
+ *
+ * Called internally after a document is returned from mongodb. Normally,
+ * you do **not** need to call this function on your own.
+ *
+ * This function triggers `init` [middleware](/docs/middleware.html).
+ * Note that `init` hooks are [synchronous](/docs/middleware.html#synchronous).
+ *
+ * @param {Object} doc document returned by mongo
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.init = function(doc, opts, fn) {
+  if (typeof opts === 'function') {
+    fn = opts;
+    opts = null;
+  }
+
+  this.$__init(doc, opts);
+
+  if (fn) {
+    fn(null, this);
+  }
+
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+Document.prototype.$__init = function(doc, opts) {
+  this.isNew = false;
+  this.$init = true;
+  opts = opts || {};
+
+  // handle docs with populated paths
+  // If doc._id is not null or undefined
+  if (doc._id != null && opts.populated && opts.populated.length) {
+    const id = String(doc._id);
+    for (const item of opts.populated) {
+      if (item.isVirtual) {
+        this.populated(item.path, utils.getValue(item.path, doc), item);
+      } else {
+        this.populated(item.path, item._docs[id], item);
+      }
+
+      if (item._childDocs == null) {
+        continue;
+      }
+      for (const child of item._childDocs) {
+        if (child == null || child.$__ == null) {
+          continue;
+        }
+        child.$__.parent = this;
+      }
+    }
+  }
+
+  init(this, doc, this._doc, opts);
+
+  markArraySubdocsPopulated(this, opts.populated);
+
+  this.emit('init', this);
+  this.constructor.emit('init', this);
+
+  this.$__._id = this._id;
+
+  return this;
+};
+
+/*!
+ * If populating a path within a document array, make sure each
+ * subdoc within the array knows its subpaths are populated.
+ *
+ * ####Example:
+ *     const doc = await Article.findOne().populate('comments.author');
+ *     doc.comments[0].populated('author'); // Should be set
+ */
+
+function markArraySubdocsPopulated(doc, populated) {
+  if (doc._id == null || populated == null || populated.length === 0) {
+    return;
+  }
+
+  const id = String(doc._id);
+  for (const item of populated) {
+    if (item.isVirtual) {
+      continue;
+    }
+    const path = item.path;
+    const pieces = path.split('.');
+    for (let i = 0; i < pieces.length - 1; ++i) {
+      const subpath = pieces.slice(0, i + 1).join('.');
+      const rest = pieces.slice(i + 1).join('.');
+      const val = doc.get(subpath);
+      if (val == null) {
+        continue;
+      }
+
+      if (val.isMongooseDocumentArray) {
+        for (let j = 0; j < val.length; ++j) {
+          val[j].populated(rest, item._docs[id] == null ? [] : item._docs[id][j], item);
+        }
+        break;
+      }
+    }
+  }
+}
+
+/*!
+ * Init helper.
+ *
+ * @param {Object} self document instance
+ * @param {Object} obj raw mongodb doc
+ * @param {Object} doc object we are initializing
+ * @api private
+ */
+
+function init(self, obj, doc, opts, prefix) {
+  prefix = prefix || '';
+
+  const keys = Object.keys(obj);
+  const len = keys.length;
+  let schema;
+  let path;
+  let i;
+  let index = 0;
+
+  while (index < len) {
+    _init(index++);
+  }
+
+  function _init(index) {
+    i = keys[index];
+    path = prefix + i;
+    schema = self.schema.path(path);
+
+    // Should still work if not a model-level discriminator, but should not be
+    // necessary. This is *only* to catch the case where we queried using the
+    // base model and the discriminated model has a projection
+    if (self.schema.$isRootDiscriminator && !self.isSelected(path)) {
+      return;
+    }
+
+    if (!schema && utils.isPOJO(obj[i])) {
+      // assume nested object
+      if (!doc[i]) {
+        doc[i] = {};
+      }
+      init(self, obj[i], doc[i], opts, path + '.');
+    } else if (!schema) {
+      doc[i] = obj[i];
+    } else {
+      if (obj[i] === null) {
+        doc[i] = schema._castNullish(null);
+      } else if (obj[i] !== undefined) {
+        const intCache = obj[i].$__ || {};
+        const wasPopulated = intCache.wasPopulated || null;
+
+        if (schema && !wasPopulated) {
+          try {
+            doc[i] = schema.cast(obj[i], self, true);
+          } catch (e) {
+            self.invalidate(e.path, new ValidatorError({
+              path: e.path,
+              message: e.message,
+              type: 'cast',
+              value: e.value,
+              reason: e
+            }));
+          }
+        } else {
+          doc[i] = obj[i];
+        }
+      }
+      // mark as hydrated
+      if (!self.isModified(path)) {
+        self.$__.activePaths.init(path);
+      }
+    }
+  }
+}
+
+/**
+ * Sends an update command with this document `_id` as the query selector.
+ *
+ * ####Example:
+ *
+ *     weirdCar.update({$inc: {wheels:1}}, { w: 1 }, callback);
+ *
+ * ####Valid options:
+ *
+ *  - same as in [Model.update](#model_Model.update)
+ *
+ * @see Model.update #model_Model.update
+ * @param {Object} doc
+ * @param {Object} options
+ * @param {Function} callback
+ * @return {Query}
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.update = function update() {
+  const args = utils.args(arguments);
+  args.unshift({ _id: this._id });
+  const query = this.constructor.update.apply(this.constructor, args);
+
+  if (this.$session() != null) {
+    if (!('session' in query.options)) {
+      query.options.session = this.$session();
+    }
+  }
+
+  return query;
+};
+
+/**
+ * Sends an updateOne command with this document `_id` as the query selector.
+ *
+ * ####Example:
+ *
+ *     weirdCar.updateOne({$inc: {wheels:1}}, { w: 1 }, callback);
+ *
+ * ####Valid options:
+ *
+ *  - same as in [Model.updateOne](#model_Model.updateOne)
+ *
+ * @see Model.updateOne #model_Model.updateOne
+ * @param {Object} doc
+ * @param {Object} [options] optional see [`Query.prototype.setOptions()`](http://mongoosejs.com/docs/api.html#query_Query-setOptions)
+ * @param {Object} [options.lean] if truthy, mongoose will return the document as a plain JavaScript object rather than a mongoose document. See [`Query.lean()`](/docs/api.html#query_Query-lean) and the [Mongoose lean tutorial](/docs/tutorials/lean.html).
+ * @param {Boolean|String} [options.strict] overwrites the schema's [strict mode option](http://mongoosejs.com/docs/guide.html#strict)
+ * @param {Boolean} [options.omitUndefined=false] If true, delete any properties whose value is `undefined` when casting an update. In other words, if this is set, Mongoose will delete `baz` from the update in `Model.updateOne({}, { foo: 'bar', baz: undefined })` before sending the update to the server.
+ * @param {Boolean} [options.timestamps=null] If set to `false` and [schema-level timestamps](/docs/guide.html#timestamps) are enabled, skip timestamps for this update. Note that this allows you to overwrite timestamps. Does nothing if schema-level timestamps are not set.
+ * @param {Function} callback
+ * @return {Query}
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.updateOne = function updateOne(doc, options, callback) {
+  const query = this.constructor.updateOne({ _id: this._id }, doc, options);
+  query._pre(cb => {
+    this.constructor._middleware.execPre('updateOne', this, [this], cb);
+  });
+  query._post(cb => {
+    this.constructor._middleware.execPost('updateOne', this, [this], {}, cb);
+  });
+
+  if (this.$session() != null) {
+    if (!('session' in query.options)) {
+      query.options.session = this.$session();
+    }
+  }
+
+  if (callback != null) {
+    return query.exec(callback);
+  }
+
+  return query;
+};
+
+/**
+ * Sends a replaceOne command with this document `_id` as the query selector.
+ *
+ * ####Valid options:
+ *
+ *  - same as in [Model.replaceOne](#model_Model.replaceOne)
+ *
+ * @see Model.replaceOne #model_Model.replaceOne
+ * @param {Object} doc
+ * @param {Object} options
+ * @param {Function} callback
+ * @return {Query}
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.replaceOne = function replaceOne() {
+  const args = utils.args(arguments);
+  args.unshift({ _id: this._id });
+  return this.constructor.replaceOne.apply(this.constructor, args);
+};
+
+/**
+ * Getter/setter around the session associated with this document. Used to
+ * automatically set `session` if you `save()` a doc that you got from a
+ * query with an associated session.
+ *
+ * ####Example:
+ *
+ *     const session = MyModel.startSession();
+ *     const doc = await MyModel.findOne().session(session);
+ *     doc.$session() === session; // true
+ *     doc.$session(null);
+ *     doc.$session() === null; // true
+ *
+ * If this is a top-level document, setting the session propagates to all child
+ * docs.
+ *
+ * @param {ClientSession} [session] overwrite the current session
+ * @return {ClientSession}
+ * @method $session
+ * @api public
+ * @memberOf Document
+ */
+
+Document.prototype.$session = function $session(session) {
+  if (arguments.length === 0) {
+    return this.$__.session;
+  }
+  this.$__.session = session;
+
+  if (!this.ownerDocument) {
+    const subdocs = this.$__getAllSubdocs();
+    for (const child of subdocs) {
+      child.$session(session);
+    }
+  }
+
+  return session;
+};
+
+/**
+ * Overwrite all values in this document with the values of `obj`, except
+ * for immutable properties. Behaves similarly to `set()`, except for it
+ * unsets all properties that aren't in `obj`.
+ *
+ * @param {Object} obj the object to overwrite this document with
+ * @method overwrite
+ * @name overwrite
+ * @memberOf Document
+ * @instance
+ * @api public
+ */
+
+Document.prototype.overwrite = function overwrite(obj) {
+  const keys = Array.from(new Set(Object.keys(this._doc).concat(Object.keys(obj))));
+
+  for (const key of keys) {
+    if (key === '_id') {
+      continue;
+    }
+    // Explicitly skip version key
+    if (this.schema.options.versionKey && key === this.schema.options.versionKey) {
+      continue;
+    }
+    if (this.schema.options.discriminatorKey && key === this.schema.options.discriminatorKey) {
+      continue;
+    }
+    this.$set(key, obj[key]);
+  }
+
+  return this;
+};
+
+/**
+ * Alias for `set()`, used internally to avoid conflicts
+ *
+ * @param {String|Object} path path or object of key/vals to set
+ * @param {Any} val the value to set
+ * @param {Schema|String|Number|Buffer|*} [type] optionally specify a type for "on-the-fly" attributes
+ * @param {Object} [options] optionally specify options that modify the behavior of the set
+ * @method $set
+ * @name $set
+ * @memberOf Document
+ * @instance
+ * @api public
+ */
+
+Document.prototype.$set = function $set(path, val, type, options) {
+  if (utils.isPOJO(type)) {
+    options = type;
+    type = undefined;
+  }
+
+  options = options || {};
+  const merge = options.merge;
+  const adhoc = type && type !== true;
+  const constructing = type === true;
+  let adhocs;
+  let keys;
+  let i = 0;
+  let pathtype;
+  let key;
+  let prefix;
+
+  const strict = 'strict' in options
+    ? options.strict
+    : this.$__.strictMode;
+
+  if (adhoc) {
+    adhocs = this.$__.adhocPaths || (this.$__.adhocPaths = {});
+    adhocs[path] = this.schema.interpretAsType(path, type, this.schema.options);
+  }
+
+  if (typeof path !== 'string') {
+    // new Document({ key: val })
+    if (path instanceof Document) {
+      if (path.$__isNested) {
+        path = path.toObject();
+      } else {
+        path = path._doc;
+      }
+    }
+
+    if (path == null) {
+      const _ = path;
+      path = val;
+      val = _;
+    } else {
+      prefix = val ? val + '.' : '';
+
+      keys = Object.keys(path);
+      const len = keys.length;
+
+      // `_skipMinimizeTopLevel` is because we may have deleted the top-level
+      // nested key to ensure key order.
+      const _skipMinimizeTopLevel = get(options, '_skipMinimizeTopLevel', false);
+      if (len === 0 && _skipMinimizeTopLevel) {
+        delete options._skipMinimizeTopLevel;
+        if (val) {
+          this.$set(val, {});
+        }
+        return this;
+      }
+
+      while (i < len) {
+        _handleIndex.call(this, i++);
+      }
+
+      return this;
+    }
+  } else {
+    this.$__.$setCalled.add(path);
+  }
+
+  function _handleIndex(i) {
+    key = keys[i];
+    const pathName = prefix + key;
+    pathtype = this.schema.pathType(pathName);
+
+    // On initial set, delete any nested keys if we're going to overwrite
+    // them to ensure we keep the user's key order.
+    if (type === true &&
+        !prefix &&
+        path[key] != null &&
+        pathtype === 'nested' &&
+        this._doc[key] != null &&
+        Object.keys(this._doc[key]).length === 0) {
+      delete this._doc[key];
+      // Make sure we set `{}` back even if we minimize re: gh-8565
+      options = Object.assign({}, options, { _skipMinimizeTopLevel: true });
+    }
+
+    if (typeof path[key] === 'object' &&
+        !utils.isNativeObject(path[key]) &&
+        !utils.isMongooseType(path[key]) &&
+        path[key] != null &&
+        pathtype !== 'virtual' &&
+        pathtype !== 'real' &&
+        pathtype !== 'adhocOrUndefined' &&
+        !(this.$__path(pathName) instanceof MixedSchema) &&
+        !(this.schema.paths[pathName] &&
+        this.schema.paths[pathName].options &&
+        this.schema.paths[pathName].options.ref)) {
+      this.$__.$setCalled.add(prefix + key);
+      this.$set(path[key], prefix + key, constructing, options);
+    } else if (strict) {
+      // Don't overwrite defaults with undefined keys (gh-3981) (gh-9039)
+      if (constructing && path[key] === void 0 &&
+          this.get(pathName) !== void 0) {
+        return;
+      }
+
+      if (pathtype === 'adhocOrUndefined') {
+        pathtype = getEmbeddedDiscriminatorPath(this, pathName, { typeOnly: true });
+      }
+
+      if (pathtype === 'real' || pathtype === 'virtual') {
+        // Check for setting single embedded schema to document (gh-3535)
+        let p = path[key];
+        if (this.schema.paths[pathName] &&
+            this.schema.paths[pathName].$isSingleNested &&
+            path[key] instanceof Document) {
+          p = p.toObject({ virtuals: false, transform: false });
+        }
+        this.$set(prefix + key, p, constructing, options);
+      } else if (pathtype === 'nested' && path[key] instanceof Document) {
+        this.$set(prefix + key,
+          path[key].toObject({ transform: false }), constructing, options);
+      } else if (strict === 'throw') {
+        if (pathtype === 'nested') {
+          throw new ObjectExpectedError(key, path[key]);
+        } else {
+          throw new StrictModeError(key);
+        }
+      }
+    } else if (path[key] !== void 0) {
+      this.$set(prefix + key, path[key], constructing, options);
+    }
+  }
+
+  let pathType = this.schema.pathType(path);
+  if (pathType === 'adhocOrUndefined') {
+    pathType = getEmbeddedDiscriminatorPath(this, path, { typeOnly: true });
+  }
+
+  // Assume this is a Mongoose document that was copied into a POJO using
+  // `Object.assign()` or `{...doc}`
+  val = handleSpreadDoc(val);
+
+  if (pathType === 'nested' && val) {
+    if (typeof val === 'object' && val != null) {
+      if (!merge) {
+        this.$__setValue(path, null);
+        cleanModifiedSubpaths(this, path);
+      } else {
+        return this.$set(val, path, constructing);
+      }
+
+      const keys = Object.keys(val);
+      this.$__setValue(path, {});
+      for (const key of keys) {
+        this.$set(path + '.' + key, val[key], constructing);
+      }
+      this.markModified(path);
+      cleanModifiedSubpaths(this, path, { skipDocArrays: true });
+      return this;
+    }
+    this.invalidate(path, new MongooseError.CastError('Object', val, path));
+    return this;
+  }
+
+  let schema;
+  const parts = path.indexOf('.') === -1 ? [path] : path.split('.');
+
+  // Might need to change path for top-level alias
+  if (typeof this.schema.aliases[parts[0]] == 'string') {
+    parts[0] = this.schema.aliases[parts[0]];
+  }
+
+  if (pathType === 'adhocOrUndefined' && strict) {
+    // check for roots that are Mixed types
+    let mixed;
+
+    for (i = 0; i < parts.length; ++i) {
+      const subpath = parts.slice(0, i + 1).join('.');
+
+      // If path is underneath a virtual, bypass everything and just set it.
+      if (i + 1 < parts.length && this.schema.pathType(subpath) === 'virtual') {
+        mpath.set(path, val, this);
+        return this;
+      }
+
+      schema = this.schema.path(subpath);
+      if (schema == null) {
+        continue;
+      }
+
+      if (schema instanceof MixedSchema) {
+        // allow changes to sub paths of mixed types
+        mixed = true;
+        break;
+      }
+    }
+
+    if (schema == null) {
+      // Check for embedded discriminators
+      schema = getEmbeddedDiscriminatorPath(this, path);
+    }
+
+    if (!mixed && !schema) {
+      if (strict === 'throw') {
+        throw new StrictModeError(path);
+      }
+      return this;
+    }
+  } else if (pathType === 'virtual') {
+    schema = this.schema.virtualpath(path);
+    schema.applySetters(val, this);
+    return this;
+  } else {
+    schema = this.$__path(path);
+  }
+
+  // gh-4578, if setting a deeply nested path that doesn't exist yet, create it
+  let cur = this._doc;
+  let curPath = '';
+  for (i = 0; i < parts.length - 1; ++i) {
+    cur = cur[parts[i]];
+    curPath += (curPath.length > 0 ? '.' : '') + parts[i];
+    if (!cur) {
+      this.$set(curPath, {});
+      // Hack re: gh-5800. If nested field is not selected, it probably exists
+      // so `MongoError: cannot use the part (nested of nested.num) to
+      // traverse the element ({nested: null})` is not likely. If user gets
+      // that error, its their fault for now. We should reconsider disallowing
+      // modifying not selected paths for 6.x
+      if (!this.isSelected(curPath)) {
+        this.unmarkModified(curPath);
+      }
+      cur = this.$__getValue(curPath);
+    }
+  }
+
+  let pathToMark;
+
+  // When using the $set operator the path to the field must already exist.
+  // Else mongodb throws: "LEFT_SUBFIELD only supports Object"
+
+  if (parts.length <= 1) {
+    pathToMark = path;
+  } else {
+    for (i = 0; i < parts.length; ++i) {
+      const subpath = parts.slice(0, i + 1).join('.');
+      if (this.get(subpath, null, { getters: false }) === null) {
+        pathToMark = subpath;
+        break;
+      }
+    }
+
+    if (!pathToMark) {
+      pathToMark = path;
+    }
+  }
+
+  // if this doc is being constructed we should not trigger getters
+  const priorVal = (() => {
+    if (this.$__.$options.priorDoc != null) {
+      return this.$__.$options.priorDoc.$__getValue(path);
+    }
+    if (constructing) {
+      return void 0;
+    }
+    return this.$__getValue(path);
+  })();
+
+  if (!schema) {
+    this.$__set(pathToMark, path, constructing, parts, schema, val, priorVal);
+    return this;
+  }
+
+  // If overwriting a subdocument path, make sure to clear out
+  // any errors _before_ setting, so new errors that happen
+  // get persisted. Re: #9080
+  if (schema.$isSingleNested || schema.$isMongooseArray) {
+    _markValidSubpaths(this, path);
+  }
+
+  if (schema.$isSingleNested && val != null && merge) {
+    if (val instanceof Document) {
+      val = val.toObject({ virtuals: false, transform: false });
+    }
+    const keys = Object.keys(val);
+    for (const key of keys) {
+      this.$set(path + '.' + key, val[key], constructing, options);
+    }
+
+    return this;
+  }
+
+  let shouldSet = true;
+  try {
+    // If the user is trying to set a ref path to a document with
+    // the correct model name, treat it as populated
+    const refMatches = (() => {
+      if (schema.options == null) {
+        return false;
+      }
+      if (!(val instanceof Document)) {
+        return false;
+      }
+      const model = val.constructor;
+
+      // Check ref
+      const ref = schema.options.ref;
+      if (ref != null && (ref === model.modelName || ref === model.baseModelName)) {
+        return true;
+      }
+
+      // Check refPath
+      const refPath = schema.options.refPath;
+      if (refPath == null) {
+        return false;
+      }
+      const modelName = val.get(refPath);
+      return modelName === model.modelName || modelName === model.baseModelName;
+    })();
+
+    let didPopulate = false;
+    if (refMatches && val instanceof Document) {
+      this.populated(path, val._id, { [populateModelSymbol]: val.constructor });
+      didPopulate = true;
+    }
+
+    let popOpts;
+    if (schema.options &&
+        Array.isArray(schema.options[this.schema.options.typeKey]) &&
+        schema.options[this.schema.options.typeKey].length &&
+        schema.options[this.schema.options.typeKey][0].ref &&
+        _isManuallyPopulatedArray(val, schema.options[this.schema.options.typeKey][0].ref)) {
+      if (this.ownerDocument) {
+        popOpts = { [populateModelSymbol]: val[0].constructor };
+        this.ownerDocument().populated(this.$__fullPath(path),
+          val.map(function(v) { return v._id; }), popOpts);
+      } else {
+        popOpts = { [populateModelSymbol]: val[0].constructor };
+        this.populated(path, val.map(function(v) { return v._id; }), popOpts);
+      }
+      didPopulate = true;
+    }
+
+    if (this.schema.singleNestedPaths[path] == null) {
+      // If this path is underneath a single nested schema, we'll call the setter
+      // later in `$__set()` because we don't take `_doc` when we iterate through
+      // a single nested doc. That's to make sure we get the correct context.
+      // Otherwise we would double-call the setter, see gh-7196.
+      val = schema.applySetters(val, this, false, priorVal);
+    }
+
+    if (schema.$isMongooseDocumentArray &&
+        Array.isArray(val) &&
+        val.length > 0 &&
+        val[0] != null &&
+        val[0].$__ != null &&
+        val[0].$__.populated != null) {
+      const populatedPaths = Object.keys(val[0].$__.populated);
+      for (const populatedPath of populatedPaths) {
+        this.populated(path + '.' + populatedPath,
+          val.map(v => v.populated(populatedPath)),
+          val[0].$__.populated[populatedPath].options);
+      }
+      didPopulate = true;
+    }
+
+    if (!didPopulate && this.$__.populated) {
+      // If this array partially contains populated documents, convert them
+      // all to ObjectIds re: #8443
+      if (Array.isArray(val) && this.$__.populated[path]) {
+        for (let i = 0; i < val.length; ++i) {
+          if (val[i] instanceof Document) {
+            val[i] = val[i]._id;
+          }
+        }
+      }
+      delete this.$__.populated[path];
+    }
+
+    if (schema.$isSingleNested && val != null) {
+      _checkImmutableSubpaths(val, schema, priorVal);
+    }
+
+    this.$markValid(path);
+  } catch (e) {
+    if (e instanceof MongooseError.StrictModeError && e.isImmutableError) {
+      this.invalidate(path, e);
+    } else if (e instanceof MongooseError.CastError) {
+      this.invalidate(e.path, e);
+      if (e.$originalErrorPath) {
+        this.invalidate(path,
+          new MongooseError.CastError(schema.instance, val, path, e.$originalErrorPath));
+      }
+    } else {
+      this.invalidate(path,
+        new MongooseError.CastError(schema.instance, val, path, e));
+    }
+    shouldSet = false;
+  }
+
+  if (shouldSet) {
+    this.$__set(pathToMark, path, constructing, parts, schema, val, priorVal);
+  }
+
+  if (schema.$isSingleNested && (this.isDirectModified(path) || val == null)) {
+    cleanModifiedSubpaths(this, path);
+  }
+
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+function _isManuallyPopulatedArray(val, ref) {
+  if (!Array.isArray(val)) {
+    return false;
+  }
+  if (val.length === 0) {
+    return false;
+  }
+
+  for (const el of val) {
+    if (!(el instanceof Document)) {
+      return false;
+    }
+    const modelName = el.constructor.modelName;
+    if (modelName == null) {
+      return false;
+    }
+    if (el.constructor.modelName != ref && el.constructor.baseModelName != ref) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+/**
+ * Sets the value of a path, or many paths.
+ *
+ * ####Example:
+ *
+ *     // path, value
+ *     doc.set(path, value)
+ *
+ *     // object
+ *     doc.set({
+ *         path  : value
+ *       , path2 : {
+ *            path  : value
+ *         }
+ *     })
+ *
+ *     // on-the-fly cast to number
+ *     doc.set(path, value, Number)
+ *
+ *     // on-the-fly cast to string
+ *     doc.set(path, value, String)
+ *
+ *     // changing strict mode behavior
+ *     doc.set(path, value, { strict: false });
+ *
+ * @param {String|Object} path path or object of key/vals to set
+ * @param {Any} val the value to set
+ * @param {Schema|String|Number|Buffer|*} [type] optionally specify a type for "on-the-fly" attributes
+ * @param {Object} [options] optionally specify options that modify the behavior of the set
+ * @api public
+ * @method set
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.set = Document.prototype.$set;
+
+/**
+ * Determine if we should mark this change as modified.
+ *
+ * @return {Boolean}
+ * @api private
+ * @method $__shouldModify
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__shouldModify = function(pathToMark, path, constructing, parts, schema, val, priorVal) {
+  if (this.isNew) {
+    return true;
+  }
+
+  // Re: the note about gh-7196, `val` is the raw value without casting or
+  // setters if the full path is under a single nested subdoc because we don't
+  // want to double run setters. So don't set it as modified. See gh-7264.
+  if (this.schema.singleNestedPaths[path] != null) {
+    return false;
+  }
+
+  if (val === void 0 && !this.isSelected(path)) {
+    // when a path is not selected in a query, its initial
+    // value will be undefined.
+    return true;
+  }
+
+  if (val === void 0 && path in this.$__.activePaths.states.default) {
+    // we're just unsetting the default value which was never saved
+    return false;
+  }
+
+  // gh-3992: if setting a populated field to a doc, don't mark modified
+  // if they have the same _id
+  if (this.populated(path) &&
+      val instanceof Document &&
+      deepEqual(val._id, priorVal)) {
+    return false;
+  }
+
+  if (!deepEqual(val, priorVal || utils.getValue(path, this))) {
+    return true;
+  }
+
+  if (!constructing &&
+      val !== null &&
+      val !== undefined &&
+      path in this.$__.activePaths.states.default &&
+      deepEqual(val, schema.getDefault(this, constructing))) {
+    // a path with a default was $unset on the server
+    // and the user is setting it to the same value again
+    return true;
+  }
+  return false;
+};
+
+/**
+ * Handles the actual setting of the value and marking the path modified if appropriate.
+ *
+ * @api private
+ * @method $__set
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__set = function(pathToMark, path, constructing, parts, schema, val, priorVal) {
+  Embedded = Embedded || require('./types/embedded');
+
+  const shouldModify = this.$__shouldModify(pathToMark, path, constructing, parts,
+    schema, val, priorVal);
+  const _this = this;
+
+  if (shouldModify) {
+    this.markModified(pathToMark);
+
+    // handle directly setting arrays (gh-1126)
+    MongooseArray || (MongooseArray = require('./types/array'));
+    if (val && val.isMongooseArray) {
+      val._registerAtomic('$set', val);
+
+      // Update embedded document parent references (gh-5189)
+      if (val.isMongooseDocumentArray) {
+        val.forEach(function(item) {
+          item && item.__parentArray && (item.__parentArray = val);
+        });
+      }
+
+      // Small hack for gh-1638: if we're overwriting the entire array, ignore
+      // paths that were modified before the array overwrite
+      this.$__.activePaths.forEach(function(modifiedPath) {
+        if (modifiedPath.startsWith(path + '.')) {
+          _this.$__.activePaths.ignore(modifiedPath);
+        }
+      });
+    }
+  }
+
+  let obj = this._doc;
+  let i = 0;
+  const l = parts.length;
+  let cur = '';
+
+  for (; i < l; i++) {
+    const next = i + 1;
+    const last = next === l;
+    cur += (cur ? '.' + parts[i] : parts[i]);
+    if (specialProperties.has(parts[i])) {
+      return;
+    }
+
+    if (last) {
+      if (obj instanceof Map) {
+        obj.set(parts[i], val);
+      } else {
+        obj[parts[i]] = val;
+      }
+    } else {
+      if (utils.isPOJO(obj[parts[i]])) {
+        obj = obj[parts[i]];
+      } else if (obj[parts[i]] && obj[parts[i]] instanceof Embedded) {
+        obj = obj[parts[i]];
+      } else if (obj[parts[i]] && obj[parts[i]].$isSingleNested) {
+        obj = obj[parts[i]];
+      } else if (obj[parts[i]] && Array.isArray(obj[parts[i]])) {
+        obj = obj[parts[i]];
+      } else {
+        obj[parts[i]] = obj[parts[i]] || {};
+        obj = obj[parts[i]];
+      }
+    }
+  }
+};
+
+/**
+ * Gets a raw value from a path (no getters)
+ *
+ * @param {String} path
+ * @api private
+ */
+
+Document.prototype.$__getValue = function(path) {
+  return utils.getValue(path, this._doc);
+};
+
+/**
+ * Sets a raw value for a path (no casting, setters, transformations)
+ *
+ * @param {String} path
+ * @param {Object} value
+ * @api private
+ */
+
+Document.prototype.$__setValue = function(path, val) {
+  utils.setValue(path, val, this._doc);
+  return this;
+};
+
+/**
+ * Returns the value of a path.
+ *
+ * ####Example
+ *
+ *     // path
+ *     doc.get('age') // 47
+ *
+ *     // dynamic casting to a string
+ *     doc.get('age', String) // "47"
+ *
+ * @param {String} path
+ * @param {Schema|String|Number|Buffer|*} [type] optionally specify a type for on-the-fly attributes
+ * @param {Object} [options]
+ * @param {Boolean} [options.virtuals=false] Apply virtuals before getting this path
+ * @param {Boolean} [options.getters=true] If false, skip applying getters and just get the raw value
+ * @api public
+ */
+
+Document.prototype.get = function(path, type, options) {
+  let adhoc;
+  options = options || {};
+  if (type) {
+    adhoc = this.schema.interpretAsType(path, type, this.schema.options);
+  }
+
+  let schema = this.$__path(path);
+  if (schema == null) {
+    schema = this.schema.virtualpath(path);
+  }
+  if (schema instanceof MixedSchema) {
+    const virtual = this.schema.virtualpath(path);
+    if (virtual != null) {
+      schema = virtual;
+    }
+  }
+  const pieces = path.split('.');
+  let obj = this._doc;
+
+  if (schema instanceof VirtualType) {
+    if (schema.getters.length === 0) {
+      return void 0;
+    }
+    return schema.applyGetters(null, this);
+  }
+
+  // Might need to change path for top-level alias
+  if (typeof this.schema.aliases[pieces[0]] == 'string') {
+    pieces[0] = this.schema.aliases[pieces[0]];
+  }
+
+  for (let i = 0, l = pieces.length; i < l; i++) {
+    if (obj && obj._doc) {
+      obj = obj._doc;
+    }
+
+    if (obj == null) {
+      obj = void 0;
+    } else if (obj instanceof Map) {
+      obj = obj.get(pieces[i], { getters: false });
+    } else if (i === l - 1) {
+      obj = utils.getValue(pieces[i], obj);
+    } else {
+      obj = obj[pieces[i]];
+    }
+  }
+
+  if (adhoc) {
+    obj = adhoc.cast(obj);
+  }
+
+  if (schema != null && options.getters !== false) {
+    obj = schema.applyGetters(obj, this);
+  } else if (this.schema.nested[path] && options.virtuals) {
+    // Might need to apply virtuals if this is a nested path
+    return applyVirtuals(this, utils.clone(obj) || {}, { path: path });
+  }
+
+  return obj;
+};
+
+/*!
+ * ignore
+ */
+
+Document.prototype[getSymbol] = Document.prototype.get;
+
+/**
+ * Returns the schematype for the given `path`.
+ *
+ * @param {String} path
+ * @api private
+ * @method $__path
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__path = function(path) {
+  const adhocs = this.$__.adhocPaths;
+  const adhocType = adhocs && adhocs.hasOwnProperty(path) ? adhocs[path] : null;
+
+  if (adhocType) {
+    return adhocType;
+  }
+  return this.schema.path(path);
+};
+
+/**
+ * Marks the path as having pending changes to write to the db.
+ *
+ * _Very helpful when using [Mixed](./schematypes.html#mixed) types._
+ *
+ * ####Example:
+ *
+ *     doc.mixed.type = 'changed';
+ *     doc.markModified('mixed.type');
+ *     doc.save() // changes to mixed.type are now persisted
+ *
+ * @param {String} path the path to mark modified
+ * @param {Document} [scope] the scope to run validators with
+ * @api public
+ */
+
+Document.prototype.markModified = function(path, scope) {
+  this.$__.activePaths.modify(path);
+  if (scope != null && !this.ownerDocument) {
+    this.$__.pathsToScopes[path] = scope;
+  }
+};
+
+/**
+ * Clears the modified state on the specified path.
+ *
+ * ####Example:
+ *
+ *     doc.foo = 'bar';
+ *     doc.unmarkModified('foo');
+ *     doc.save(); // changes to foo will not be persisted
+ *
+ * @param {String} path the path to unmark modified
+ * @api public
+ */
+
+Document.prototype.unmarkModified = function(path) {
+  this.$__.activePaths.init(path);
+  delete this.$__.pathsToScopes[path];
+};
+
+/**
+ * Don't run validation on this path or persist changes to this path.
+ *
+ * ####Example:
+ *
+ *     doc.foo = null;
+ *     doc.$ignore('foo');
+ *     doc.save(); // changes to foo will not be persisted and validators won't be run
+ *
+ * @memberOf Document
+ * @instance
+ * @method $ignore
+ * @param {String} path the path to ignore
+ * @api public
+ */
+
+Document.prototype.$ignore = function(path) {
+  this.$__.activePaths.ignore(path);
+};
+
+/**
+ * Returns the list of paths that have been directly modified. A direct
+ * modified path is a path that you explicitly set, whether via `doc.foo = 'bar'`,
+ * `Object.assign(doc, { foo: 'bar' })`, or `doc.set('foo', 'bar')`.
+ *
+ * A path `a` may be in `modifiedPaths()` but not in `directModifiedPaths()`
+ * because a child of `a` was directly modified.
+ *
+ * ####Example
+ *     const schema = new Schema({ foo: String, nested: { bar: String } });
+ *     const Model = mongoose.model('Test', schema);
+ *     await Model.create({ foo: 'original', nested: { bar: 'original' } });
+ *
+ *     const doc = await Model.findOne();
+ *     doc.nested.bar = 'modified';
+ *     doc.directModifiedPaths(); // ['nested.bar']
+ *     doc.modifiedPaths(); // ['nested', 'nested.bar']
+ *
+ * @return {Array}
+ * @api public
+ */
+
+Document.prototype.directModifiedPaths = function() {
+  return Object.keys(this.$__.activePaths.states.modify);
+};
+
+/**
+ * Returns true if the given path is nullish or only contains empty objects.
+ * Useful for determining whether this subdoc will get stripped out by the
+ * [minimize option](/docs/guide.html#minimize).
+ *
+ * ####Example:
+ *     const schema = new Schema({ nested: { foo: String } });
+ *     const Model = mongoose.model('Test', schema);
+ *     const doc = new Model({});
+ *     doc.$isEmpty('nested'); // true
+ *     doc.nested.$isEmpty(); // true
+ *
+ *     doc.nested.foo = 'bar';
+ *     doc.$isEmpty('nested'); // false
+ *     doc.nested.$isEmpty(); // false
+ *
+ * @memberOf Document
+ * @instance
+ * @api public
+ * @method $isEmpty
+ * @return {Boolean}
+ */
+
+Document.prototype.$isEmpty = function(path) {
+  const isEmptyOptions = {
+    minimize: true,
+    virtuals: false,
+    getters: false,
+    transform: false
+  };
+
+  if (arguments.length > 0) {
+    const v = this.get(path);
+    if (v == null) {
+      return true;
+    }
+    if (typeof v !== 'object') {
+      return false;
+    }
+    if (utils.isPOJO(v)) {
+      return _isEmpty(v);
+    }
+    return Object.keys(v.toObject(isEmptyOptions)).length === 0;
+  }
+
+  return Object.keys(this.toObject(isEmptyOptions)).length === 0;
+};
+
+function _isEmpty(v) {
+  if (v == null) {
+    return true;
+  }
+  if (typeof v !== 'object' || Array.isArray(v)) {
+    return false;
+  }
+  for (const key of Object.keys(v)) {
+    if (!_isEmpty(v[key])) {
+      return false;
+    }
+  }
+  return true;
+}
+
+/**
+ * Returns the list of paths that have been modified.
+ *
+ * @param {Object} [options]
+ * @param {Boolean} [options.includeChildren=false] if true, returns children of modified paths as well. For example, if false, the list of modified paths for `doc.colors = { primary: 'blue' };` will **not** contain `colors.primary`. If true, `modifiedPaths()` will return an array that contains `colors.primary`.
+ * @return {Array}
+ * @api public
+ */
+
+Document.prototype.modifiedPaths = function(options) {
+  options = options || {};
+  const directModifiedPaths = Object.keys(this.$__.activePaths.states.modify);
+  const _this = this;
+  return directModifiedPaths.reduce(function(list, path) {
+    const parts = path.split('.');
+    list = list.concat(parts.reduce(function(chains, part, i) {
+      return chains.concat(parts.slice(0, i).concat(part).join('.'));
+    }, []).filter(function(chain) {
+      return (list.indexOf(chain) === -1);
+    }));
+
+    if (!options.includeChildren) {
+      return list;
+    }
+
+    let cur = _this.get(path);
+    if (cur != null && typeof cur === 'object') {
+      if (cur._doc) {
+        cur = cur._doc;
+      }
+      if (Array.isArray(cur)) {
+        const len = cur.length;
+        for (let i = 0; i < len; ++i) {
+          if (list.indexOf(path + '.' + i) === -1) {
+            list.push(path + '.' + i);
+            if (cur[i] != null && cur[i].$__) {
+              const modified = cur[i].modifiedPaths();
+              for (const childPath of modified) {
+                list.push(path + '.' + i + '.' + childPath);
+              }
+            }
+          }
+        }
+      } else {
+        Object.keys(cur).
+          filter(function(key) {
+            return list.indexOf(path + '.' + key) === -1;
+          }).
+          forEach(function(key) {
+            list.push(path + '.' + key);
+          });
+      }
+    }
+
+    return list;
+  }, []);
+};
+
+Document.prototype[documentModifiedPaths] = Document.prototype.modifiedPaths;
+
+/**
+ * Returns true if this document was modified, else false.
+ *
+ * If `path` is given, checks if a path or any full path containing `path` as part of its path chain has been modified.
+ *
+ * ####Example
+ *
+ *     doc.set('documents.0.title', 'changed');
+ *     doc.isModified()                      // true
+ *     doc.isModified('documents')           // true
+ *     doc.isModified('documents.0.title')   // true
+ *     doc.isModified('documents otherProp') // true
+ *     doc.isDirectModified('documents')     // false
+ *
+ * @param {String} [path] optional
+ * @return {Boolean}
+ * @api public
+ */
+
+Document.prototype.isModified = function(paths, modifiedPaths) {
+  if (paths) {
+    if (!Array.isArray(paths)) {
+      paths = paths.split(' ');
+    }
+    const modified = modifiedPaths || this[documentModifiedPaths]();
+    const directModifiedPaths = Object.keys(this.$__.activePaths.states.modify);
+    const isModifiedChild = paths.some(function(path) {
+      return !!~modified.indexOf(path);
+    });
+
+    return isModifiedChild || paths.some(function(path) {
+      return directModifiedPaths.some(function(mod) {
+        return mod === path || path.startsWith(mod + '.');
+      });
+    });
+  }
+
+  return this.$__.activePaths.some('modify');
+};
+
+Document.prototype[documentIsModified] = Document.prototype.isModified;
+
+/**
+ * Checks if a path is set to its default.
+ *
+ * ####Example
+ *
+ *     MyModel = mongoose.model('test', { name: { type: String, default: 'Val '} });
+ *     const m = new MyModel();
+ *     m.$isDefault('name'); // true
+ *
+ * @memberOf Document
+ * @instance
+ * @method $isDefault
+ * @param {String} [path]
+ * @return {Boolean}
+ * @api public
+ */
+
+Document.prototype.$isDefault = function(path) {
+  return (path in this.$__.activePaths.states.default);
+};
+
+/**
+ * Getter/setter, determines whether the document was removed or not.
+ *
+ * ####Example:
+ *     product.remove(function (err, product) {
+ *       product.$isDeleted(); // true
+ *       product.remove(); // no-op, doesn't send anything to the db
+ *
+ *       product.$isDeleted(false);
+ *       product.$isDeleted(); // false
+ *       product.remove(); // will execute a remove against the db
+ *     })
+ *
+ * @param {Boolean} [val] optional, overrides whether mongoose thinks the doc is deleted
+ * @return {Boolean} whether mongoose thinks this doc is deleted.
+ * @method $isDeleted
+ * @memberOf Document
+ * @instance
+ * @api public
+ */
+
+Document.prototype.$isDeleted = function(val) {
+  if (arguments.length === 0) {
+    return !!this.$__.isDeleted;
+  }
+
+  this.$__.isDeleted = !!val;
+  return this;
+};
+
+/**
+ * Returns true if `path` was directly set and modified, else false.
+ *
+ * ####Example
+ *
+ *     doc.set('documents.0.title', 'changed');
+ *     doc.isDirectModified('documents.0.title') // true
+ *     doc.isDirectModified('documents') // false
+ *
+ * @param {String} path
+ * @return {Boolean}
+ * @api public
+ */
+
+Document.prototype.isDirectModified = function(path) {
+  return (path in this.$__.activePaths.states.modify);
+};
+
+/**
+ * Checks if `path` was initialized.
+ *
+ * @param {String} path
+ * @return {Boolean}
+ * @api public
+ */
+
+Document.prototype.isInit = function(path) {
+  return (path in this.$__.activePaths.states.init);
+};
+
+/**
+ * Checks if `path` was selected in the source query which initialized this document.
+ *
+ * ####Example
+ *
+ *     Thing.findOne().select('name').exec(function (err, doc) {
+ *        doc.isSelected('name') // true
+ *        doc.isSelected('age')  // false
+ *     })
+ *
+ * @param {String} path
+ * @return {Boolean}
+ * @api public
+ */
+
+Document.prototype.isSelected = function isSelected(path) {
+  if (this.$__.selected) {
+    if (path === '_id') {
+      return this.$__.selected._id !== 0;
+    }
+
+    const paths = Object.keys(this.$__.selected);
+    let i = paths.length;
+    let inclusive = null;
+    let cur;
+
+    if (i === 1 && paths[0] === '_id') {
+      // only _id was selected.
+      return this.$__.selected._id === 0;
+    }
+
+    while (i--) {
+      cur = paths[i];
+      if (cur === '_id') {
+        continue;
+      }
+      if (!isDefiningProjection(this.$__.selected[cur])) {
+        continue;
+      }
+      inclusive = !!this.$__.selected[cur];
+      break;
+    }
+
+    if (inclusive === null) {
+      return true;
+    }
+
+    if (path in this.$__.selected) {
+      return inclusive;
+    }
+
+    i = paths.length;
+    const pathDot = path + '.';
+
+    while (i--) {
+      cur = paths[i];
+      if (cur === '_id') {
+        continue;
+      }
+
+      if (cur.startsWith(pathDot)) {
+        return inclusive || cur !== pathDot;
+      }
+
+      if (pathDot.startsWith(cur + '.')) {
+        return inclusive;
+      }
+    }
+
+    return !inclusive;
+  }
+
+  return true;
+};
+
+Document.prototype[documentIsSelected] = Document.prototype.isSelected;
+
+/**
+ * Checks if `path` was explicitly selected. If no projection, always returns
+ * true.
+ *
+ * ####Example
+ *
+ *     Thing.findOne().select('nested.name').exec(function (err, doc) {
+ *        doc.isDirectSelected('nested.name') // true
+ *        doc.isDirectSelected('nested.otherName') // false
+ *        doc.isDirectSelected('nested')  // false
+ *     })
+ *
+ * @param {String} path
+ * @return {Boolean}
+ * @api public
+ */
+
+Document.prototype.isDirectSelected = function isDirectSelected(path) {
+  if (this.$__.selected) {
+    if (path === '_id') {
+      return this.$__.selected._id !== 0;
+    }
+
+    const paths = Object.keys(this.$__.selected);
+    let i = paths.length;
+    let inclusive = null;
+    let cur;
+
+    if (i === 1 && paths[0] === '_id') {
+      // only _id was selected.
+      return this.$__.selected._id === 0;
+    }
+
+    while (i--) {
+      cur = paths[i];
+      if (cur === '_id') {
+        continue;
+      }
+      if (!isDefiningProjection(this.$__.selected[cur])) {
+        continue;
+      }
+      inclusive = !!this.$__.selected[cur];
+      break;
+    }
+
+    if (inclusive === null) {
+      return true;
+    }
+
+    if (path in this.$__.selected) {
+      return inclusive;
+    }
+
+    return !inclusive;
+  }
+
+  return true;
+};
+
+/**
+ * Executes registered validation rules for this document.
+ *
+ * ####Note:
+ *
+ * This method is called `pre` save and if a validation rule is violated, [save](#model_Model-save) is aborted and the error is returned to your `callback`.
+ *
+ * ####Example:
+ *
+ *     doc.validate(function (err) {
+ *       if (err) handleError(err);
+ *       else // validation passed
+ *     });
+ *
+ * @param {Array|String} [pathsToValidate] list of paths to validate. If set, Mongoose will validate only the modified paths that are in the given list.
+ * @param {Object} [options] internal options
+ * @param {Boolean} [options.validateModifiedOnly=false] if `true` mongoose validates only modified paths.
+ * @param {Function} [callback] optional callback called after validation completes, passing an error if one occurred
+ * @return {Promise} Promise
+ * @api public
+ */
+
+Document.prototype.validate = function(pathsToValidate, options, callback) {
+  let parallelValidate;
+  this.$op = 'validate';
+
+  if (this.ownerDocument != null) {
+    // Skip parallel validate check for subdocuments
+  } else if (this.$__.validating) {
+    parallelValidate = new ParallelValidateError(this, {
+      parentStack: options && options.parentStack,
+      conflictStack: this.$__.validating.stack
+    });
+  } else {
+    this.$__.validating = new ParallelValidateError(this, { parentStack: options && options.parentStack });
+  }
+
+  if (typeof pathsToValidate === 'function') {
+    callback = pathsToValidate;
+    options = null;
+    pathsToValidate = null;
+  } else if (typeof options === 'function') {
+    callback = options;
+    options = pathsToValidate;
+    pathsToValidate = null;
+  }
+
+  return promiseOrCallback(callback, cb => {
+    if (parallelValidate != null) {
+      return cb(parallelValidate);
+    }
+
+    this.$__validate(pathsToValidate, options, (error) => {
+      this.$op = null;
+      cb(error);
+    });
+  }, this.constructor.events);
+};
+
+/*!
+ * ignore
+ */
+
+function _evaluateRequiredFunctions(doc) {
+  Object.keys(doc.$__.activePaths.states.require).forEach(path => {
+    const p = doc.schema.path(path);
+
+    if (p != null && typeof p.originalRequiredValue === 'function') {
+      doc.$__.cachedRequired[path] = p.originalRequiredValue.call(doc, doc);
+    }
+  });
+}
+
+/*!
+ * ignore
+ */
+
+function _getPathsToValidate(doc) {
+  const skipSchemaValidators = {};
+
+  _evaluateRequiredFunctions(doc);
+
+  // only validate required fields when necessary
+  let paths = new Set(Object.keys(doc.$__.activePaths.states.require).filter(function(path) {
+    if (!doc.isSelected(path) && !doc.isModified(path)) {
+      return false;
+    }
+    if (path in doc.$__.cachedRequired) {
+      return doc.$__.cachedRequired[path];
+    }
+    return true;
+  }));
+
+
+  Object.keys(doc.$__.activePaths.states.init).forEach(addToPaths);
+  Object.keys(doc.$__.activePaths.states.modify).forEach(addToPaths);
+  Object.keys(doc.$__.activePaths.states.default).forEach(addToPaths);
+  function addToPaths(p) { paths.add(p); }
+
+  const subdocs = doc.$__getAllSubdocs();
+  const modifiedPaths = doc.modifiedPaths();
+  for (const subdoc of subdocs) {
+    if (subdoc.$basePath) {
+      // Remove child paths for now, because we'll be validating the whole
+      // subdoc
+      for (const p of paths) {
+        if (p === null || p.startsWith(subdoc.$basePath + '.')) {
+          paths.delete(p);
+        }
+      }
+
+      if (doc.isModified(subdoc.$basePath, modifiedPaths) &&
+            !doc.isDirectModified(subdoc.$basePath) &&
+            !doc.$isDefault(subdoc.$basePath)) {
+        paths.add(subdoc.$basePath);
+
+        skipSchemaValidators[subdoc.$basePath] = true;
+      }
+    }
+  }
+
+  // from here on we're not removing items from paths
+
+  // gh-661: if a whole array is modified, make sure to run validation on all
+  // the children as well
+  for (const path of paths) {
+    const _pathType = doc.schema.path(path);
+    if (!_pathType ||
+        !_pathType.$isMongooseArray ||
+        // To avoid potential performance issues, skip doc arrays whose children
+        // are not required. `getPositionalPathType()` may be slow, so avoid
+        // it unless we have a case of #6364
+        (_pathType.$isMongooseDocumentArray && !get(_pathType, 'schemaOptions.required'))) {
+      continue;
+    }
+
+    const val = doc.$__getValue(path);
+    _pushNestedArrayPaths(val, paths, path);
+  }
+
+  function _pushNestedArrayPaths(val, paths, path) {
+    if (val != null) {
+      const numElements = val.length;
+      for (let j = 0; j < numElements; ++j) {
+        if (Array.isArray(val[j])) {
+          _pushNestedArrayPaths(val[j], paths, path + '.' + j);
+        } else {
+          paths.add(path + '.' + j);
+        }
+      }
+    }
+  }
+
+  const flattenOptions = { skipArrays: true };
+  for (const pathToCheck of paths) {
+    if (doc.schema.nested[pathToCheck]) {
+      let _v = doc.$__getValue(pathToCheck);
+      if (isMongooseObject(_v)) {
+        _v = _v.toObject({ transform: false });
+      }
+      const flat = flatten(_v, pathToCheck, flattenOptions, doc.schema);
+      Object.keys(flat).forEach(addToPaths);
+    }
+  }
+
+
+  for (const path of paths) {
+    // Single nested paths (paths embedded under single nested subdocs) will
+    // be validated on their own when we call `validate()` on the subdoc itself.
+    // Re: gh-8468
+    if (doc.schema.singleNestedPaths.hasOwnProperty(path)) {
+      paths.delete(path);
+      continue;
+    }
+    const _pathType = doc.schema.path(path);
+    if (!_pathType || !_pathType.$isSchemaMap) {
+      continue;
+    }
+
+    const val = doc.$__getValue(path);
+    if (val == null) {
+      continue;
+    }
+    for (const key of val.keys()) {
+      paths.add(path + '.' + key);
+    }
+  }
+
+  paths = Array.from(paths);
+  return [paths, skipSchemaValidators];
+}
+
+/*!
+ * ignore
+ */
+
+Document.prototype.$__validate = function(pathsToValidate, options, callback) {
+  if (typeof pathsToValidate === 'function') {
+    callback = pathsToValidate;
+    options = null;
+    pathsToValidate = null;
+  } else if (typeof options === 'function') {
+    callback = options;
+    options = null;
+  }
+
+  const hasValidateModifiedOnlyOption = options &&
+      (typeof options === 'object') &&
+      ('validateModifiedOnly' in options);
+
+  let shouldValidateModifiedOnly;
+  if (hasValidateModifiedOnlyOption) {
+    shouldValidateModifiedOnly = !!options.validateModifiedOnly;
+  } else {
+    shouldValidateModifiedOnly = this.schema.options.validateModifiedOnly;
+  }
+
+  const _this = this;
+  const _complete = () => {
+    let validationError = this.$__.validationError;
+    this.$__.validationError = undefined;
+
+    if (shouldValidateModifiedOnly && validationError != null) {
+      // Remove any validation errors that aren't from modified paths
+      const errors = Object.keys(validationError.errors);
+      for (const errPath of errors) {
+        if (!this.isModified(errPath)) {
+          delete validationError.errors[errPath];
+        }
+      }
+      if (Object.keys(validationError.errors).length === 0) {
+        validationError = void 0;
+      }
+    }
+
+    this.$__.cachedRequired = {};
+    this.emit('validate', _this);
+    this.constructor.emit('validate', _this);
+
+    this.$__.validating = null;
+    if (validationError) {
+      for (const key in validationError.errors) {
+        // Make sure cast errors persist
+        if (!this[documentArrayParent] &&
+            validationError.errors[key] instanceof MongooseError.CastError) {
+          this.invalidate(key, validationError.errors[key]);
+        }
+      }
+
+      return validationError;
+    }
+  };
+
+  // only validate required fields when necessary
+  const pathDetails = _getPathsToValidate(this);
+  let paths = shouldValidateModifiedOnly ?
+    pathDetails[0].filter((path) => this.isModified(path)) :
+    pathDetails[0];
+  const skipSchemaValidators = pathDetails[1];
+
+  if (Array.isArray(pathsToValidate)) {
+    paths = _handlePathsToValidate(paths, pathsToValidate);
+  }
+
+  if (paths.length === 0) {
+    return process.nextTick(function() {
+      const error = _complete();
+      if (error) {
+        return _this.schema.s.hooks.execPost('validate:error', _this, [_this], { error: error }, function(error) {
+          callback(error);
+        });
+      }
+      callback(null, _this);
+    });
+  }
+
+  const validated = {};
+  let total = 0;
+
+  const complete = function() {
+    const error = _complete();
+    if (error) {
+      return _this.schema.s.hooks.execPost('validate:error', _this, [_this], { error: error }, function(error) {
+        callback(error);
+      });
+    }
+    callback(null, _this);
+  };
+
+  const validatePath = function(path) {
+    if (path == null || validated[path]) {
+      return;
+    }
+
+    validated[path] = true;
+    total++;
+
+    process.nextTick(function() {
+      const schemaType = _this.schema.path(path);
+
+      if (!schemaType) {
+        return --total || complete();
+      }
+
+      // If user marked as invalid or there was a cast error, don't validate
+      if (!_this.$isValid(path)) {
+        --total || complete();
+        return;
+      }
+
+      let val = _this.$__getValue(path);
+
+      // If you `populate()` and get back a null value, required validators
+      // shouldn't fail (gh-8018). We should always fall back to the populated
+      // value.
+      let pop;
+      if (val == null && (pop = _this.populated(path))) {
+        val = pop;
+      }
+      const scope = path in _this.$__.pathsToScopes ?
+        _this.$__.pathsToScopes[path] :
+        _this;
+
+      const doValidateOptions = {
+        skipSchemaValidators: skipSchemaValidators[path],
+        path: path
+      };
+      schemaType.doValidate(val, function(err) {
+        if (err && (!schemaType.$isMongooseDocumentArray || err.$isArrayValidatorError)) {
+          if (schemaType.$isSingleNested &&
+              err instanceof ValidationError &&
+              schemaType.schema.options.storeSubdocValidationError === false) {
+            return --total || complete();
+          }
+          _this.invalidate(path, err, undefined, true);
+        }
+        --total || complete();
+      }, scope, doValidateOptions);
+    });
+  };
+
+  const numPaths = paths.length;
+  for (let i = 0; i < numPaths; ++i) {
+    validatePath(paths[i]);
+  }
+};
+
+/*!
+ * ignore
+ */
+
+function _handlePathsToValidate(paths, pathsToValidate) {
+  const _pathsToValidate = new Set(pathsToValidate);
+  const parentPaths = new Map([]);
+  for (const path of pathsToValidate) {
+    if (path.indexOf('.') === -1) {
+      continue;
+    }
+    const pieces = path.split('.');
+    let cur = pieces[0];
+    for (let i = 1; i < pieces.length; ++i) {
+      // Since we skip subpaths under single nested subdocs to
+      // avoid double validation, we need to add back the
+      // single nested subpath if the user asked for it (gh-8626)
+      parentPaths.set(cur, path);
+      cur = cur + '.' + pieces[i];
+    }
+  }
+
+  const ret = [];
+  for (const path of paths) {
+    if (_pathsToValidate.has(path)) {
+      ret.push(path);
+    } else if (parentPaths.has(path)) {
+      ret.push(parentPaths.get(path));
+    }
+  }
+  return ret;
+}
+
+/**
+ * Executes registered validation rules (skipping asynchronous validators) for this document.
+ *
+ * ####Note:
+ *
+ * This method is useful if you need synchronous validation.
+ *
+ * ####Example:
+ *
+ *     const err = doc.validateSync();
+ *     if (err) {
+ *       handleError(err);
+ *     } else {
+ *       // validation passed
+ *     }
+ *
+ * @param {Array|string} pathsToValidate only validate the given paths
+ * @param {Object} [options] options for validation
+ * @param {Boolean} [options.validateModifiedOnly=false] If `true`, Mongoose will only validate modified paths, as opposed to modified paths and `required` paths.
+ * @return {ValidationError|undefined} ValidationError if there are errors during validation, or undefined if there is no error.
+ * @api public
+ */
+
+Document.prototype.validateSync = function(pathsToValidate, options) {
+  const _this = this;
+
+  const hasValidateModifiedOnlyOption = options &&
+      (typeof options === 'object') &&
+      ('validateModifiedOnly' in options);
+
+  let shouldValidateModifiedOnly;
+  if (hasValidateModifiedOnlyOption) {
+    shouldValidateModifiedOnly = !!options.validateModifiedOnly;
+  } else {
+    shouldValidateModifiedOnly = this.schema.options.validateModifiedOnly;
+  }
+
+  if (typeof pathsToValidate === 'string') {
+    pathsToValidate = pathsToValidate.split(' ');
+  }
+
+  // only validate required fields when necessary
+  const pathDetails = _getPathsToValidate(this);
+  let paths = shouldValidateModifiedOnly ?
+    pathDetails[0].filter((path) => this.isModified(path)) :
+    pathDetails[0];
+  const skipSchemaValidators = pathDetails[1];
+
+  if (Array.isArray(pathsToValidate)) {
+    paths = _handlePathsToValidate(paths, pathsToValidate);
+  }
+
+  const validating = {};
+
+  paths.forEach(function(path) {
+    if (validating[path]) {
+      return;
+    }
+
+    validating[path] = true;
+
+    const p = _this.schema.path(path);
+    if (!p) {
+      return;
+    }
+    if (!_this.$isValid(path)) {
+      return;
+    }
+
+    const val = _this.$__getValue(path);
+    const err = p.doValidateSync(val, _this, {
+      skipSchemaValidators: skipSchemaValidators[path],
+      path: path
+    });
+    if (err && (!p.$isMongooseDocumentArray || err.$isArrayValidatorError)) {
+      if (p.$isSingleNested &&
+          err instanceof ValidationError &&
+          p.schema.options.storeSubdocValidationError === false) {
+        return;
+      }
+      _this.invalidate(path, err, undefined, true);
+    }
+  });
+
+  const err = _this.$__.validationError;
+  _this.$__.validationError = undefined;
+  _this.emit('validate', _this);
+  _this.constructor.emit('validate', _this);
+
+  if (err) {
+    for (const key in err.errors) {
+      // Make sure cast errors persist
+      if (err.errors[key] instanceof MongooseError.CastError) {
+        _this.invalidate(key, err.errors[key]);
+      }
+    }
+  }
+
+  return err;
+};
+
+/**
+ * Marks a path as invalid, causing validation to fail.
+ *
+ * The `errorMsg` argument will become the message of the `ValidationError`.
+ *
+ * The `value` argument (if passed) will be available through the `ValidationError.value` property.
+ *
+ *     doc.invalidate('size', 'must be less than 20', 14);
+
+ *     doc.validate(function (err) {
+ *       console.log(err)
+ *       // prints
+ *       { message: 'Validation failed',
+ *         name: 'ValidationError',
+ *         errors:
+ *          { size:
+ *             { message: 'must be less than 20',
+ *               name: 'ValidatorError',
+ *               path: 'size',
+ *               type: 'user defined',
+ *               value: 14 } } }
+ *     })
+ *
+ * @param {String} path the field to invalidate. For array elements, use the `array.i.field` syntax, where `i` is the 0-based index in the array.
+ * @param {String|Error} errorMsg the error which states the reason `path` was invalid
+ * @param {Object|String|Number|any} value optional invalid value
+ * @param {String} [kind] optional `kind` property for the error
+ * @return {ValidationError} the current ValidationError, with all currently invalidated paths
+ * @api public
+ */
+
+Document.prototype.invalidate = function(path, err, val, kind) {
+  if (!this.$__.validationError) {
+    this.$__.validationError = new ValidationError(this);
+  }
+
+  if (this.$__.validationError.errors[path]) {
+    return;
+  }
+
+  if (!err || typeof err === 'string') {
+    err = new ValidatorError({
+      path: path,
+      message: err,
+      type: kind || 'user defined',
+      value: val
+    });
+  }
+
+  if (this.$__.validationError === err) {
+    return this.$__.validationError;
+  }
+
+  this.$__.validationError.addError(path, err);
+  return this.$__.validationError;
+};
+
+/**
+ * Marks a path as valid, removing existing validation errors.
+ *
+ * @param {String} path the field to mark as valid
+ * @api public
+ * @memberOf Document
+ * @instance
+ * @method $markValid
+ */
+
+Document.prototype.$markValid = function(path) {
+  if (!this.$__.validationError || !this.$__.validationError.errors[path]) {
+    return;
+  }
+
+  delete this.$__.validationError.errors[path];
+  if (Object.keys(this.$__.validationError.errors).length === 0) {
+    this.$__.validationError = null;
+  }
+};
+
+/*!
+ * ignore
+ */
+
+function _markValidSubpaths(doc, path) {
+  if (!doc.$__.validationError) {
+    return;
+  }
+
+  const keys = Object.keys(doc.$__.validationError.errors);
+  for (const key of keys) {
+    if (key.startsWith(path + '.')) {
+      delete doc.$__.validationError.errors[key];
+    }
+  }
+  if (Object.keys(doc.$__.validationError.errors).length === 0) {
+    doc.$__.validationError = null;
+  }
+}
+
+/*!
+ * ignore
+ */
+
+function _checkImmutableSubpaths(subdoc, schematype, priorVal) {
+  const schema = schematype.schema;
+  if (schema == null) {
+    return;
+  }
+
+  for (const key of Object.keys(schema.paths)) {
+    const path = schema.paths[key];
+    if (path.$immutableSetter == null) {
+      continue;
+    }
+    const oldVal = priorVal == null ? void 0 : priorVal.$__getValue(key);
+    // Calling immutableSetter with `oldVal` even though it expects `newVal`
+    // is intentional. That's because `$immutableSetter` compares its param
+    // to the current value.
+    path.$immutableSetter.call(subdoc, oldVal);
+  }
+}
+
+/**
+ * Saves this document by inserting a new document into the database if [document.isNew](/docs/api.html#document_Document-isNew) is `true`,
+ * or sends an [updateOne](/docs/api.html#document_Document-updateOne) operation **only** with the modifications to the database, it does not replace the whole document in the latter case.
+ *
+ * ####Example:
+ *
+ *     product.sold = Date.now();
+ *     product = await product.save();
+ *
+ * If save is successful, the returned promise will fulfill with the document
+ * saved.
+ *
+ * ####Example:
+ *
+ *     const newProduct = await product.save();
+ *     newProduct === product; // true
+ *
+ * @param {Object} [options] options optional options
+ * @param {Session} [options.session=null] the [session](https://docs.mongodb.com/manual/reference/server-sessions/) associated with this save operation. If not specified, defaults to the [document's associated session](api.html#document_Document-$session).
+ * @param {Object} [options.safe] (DEPRECATED) overrides [schema's safe option](http://mongoosejs.com//docs/guide.html#safe). Use the `w` option instead.
+ * @param {Boolean} [options.validateBeforeSave] set to false to save without validating.
+ * @param {Boolean} [options.validateModifiedOnly=false] If `true`, Mongoose will only validate modified paths, as opposed to modified paths and `required` paths.
+ * @param {Number|String} [options.w] set the [write concern](https://docs.mongodb.com/manual/reference/write-concern/#w-option). Overrides the [schema-level `writeConcern` option](/docs/guide.html#writeConcern)
+ * @param {Boolean} [options.j] set to true for MongoDB to wait until this `save()` has been [journaled before resolving the returned promise](https://docs.mongodb.com/manual/reference/write-concern/#j-option). Overrides the [schema-level `writeConcern` option](/docs/guide.html#writeConcern)
+ * @param {Number} [options.wtimeout] sets a [timeout for the write concern](https://docs.mongodb.com/manual/reference/write-concern/#wtimeout). Overrides the [schema-level `writeConcern` option](/docs/guide.html#writeConcern).
+ * @param {Boolean} [options.checkKeys=true] the MongoDB driver prevents you from saving keys that start with '$' or contain '.' by default. Set this option to `false` to skip that check. See [restrictions on field names](https://docs.mongodb.com/manual/reference/limits/#Restrictions-on-Field-Names)
+ * @param {Boolean} [options.timestamps=true] if `false` and [timestamps](./guide.html#timestamps) are enabled, skip timestamps for this `save()`.
+ * @param {Function} [fn] optional callback
+ * @method save
+ * @memberOf Document
+ * @instance
+ * @throws {DocumentNotFoundError} if this [save updates an existing document](api.html#document_Document-isNew) but the document doesn't exist in the database. For example, you will get this error if the document is [deleted between when you retrieved the document and when you saved it](documents.html#updating).
+ * @return {Promise|undefined} Returns undefined if used with callback or a Promise otherwise.
+ * @api public
+ * @see middleware http://mongoosejs.com/docs/middleware.html
+ */
+
+/**
+ * Checks if a path is invalid
+ *
+ * @param {String} path the field to check
+ * @method $isValid
+ * @memberOf Document
+ * @instance
+ * @api private
+ */
+
+Document.prototype.$isValid = function(path) {
+  return !this.$__.validationError || !this.$__.validationError.errors[path];
+};
+
+/**
+ * Resets the internal modified state of this document.
+ *
+ * @api private
+ * @return {Document}
+ * @method $__reset
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__reset = function reset() {
+  let _this = this;
+  DocumentArray || (DocumentArray = require('./types/documentarray'));
+
+  this.$__.activePaths
+    .map('init', 'modify', function(i) {
+      return _this.$__getValue(i);
+    })
+    .filter(function(val) {
+      return val && val instanceof Array && val.isMongooseDocumentArray && val.length;
+    })
+    .forEach(function(array) {
+      let i = array.length;
+      while (i--) {
+        const doc = array[i];
+        if (!doc) {
+          continue;
+        }
+        doc.$__reset();
+      }
+
+      _this.$__.activePaths.init(array.$path());
+
+      array[arrayAtomicsBackupSymbol] = array[arrayAtomicsSymbol];
+      array[arrayAtomicsSymbol] = {};
+    });
+
+  this.$__.activePaths.
+    map('init', 'modify', function(i) {
+      return _this.$__getValue(i);
+    }).
+    filter(function(val) {
+      return val && val.$isSingleNested;
+    }).
+    forEach(function(doc) {
+      doc.$__reset();
+      _this.$__.activePaths.init(doc.$basePath);
+    });
+
+  // clear atomics
+  this.$__dirty().forEach(function(dirt) {
+    const type = dirt.value;
+
+    if (type && type[arrayAtomicsSymbol]) {
+      type[arrayAtomicsBackupSymbol] = type[arrayAtomicsSymbol];
+      type[arrayAtomicsSymbol] = {};
+    }
+  });
+
+  this.$__.backup = {};
+  this.$__.backup.activePaths = {
+    modify: Object.assign({}, this.$__.activePaths.states.modify),
+    default: Object.assign({}, this.$__.activePaths.states.default)
+  };
+  this.$__.backup.validationError = this.$__.validationError;
+  this.$__.backup.errors = this.errors;
+
+  // Clear 'dirty' cache
+  this.$__.activePaths.clear('modify');
+  this.$__.activePaths.clear('default');
+  this.$__.validationError = undefined;
+  this.errors = undefined;
+  _this = this;
+  this.schema.requiredPaths().forEach(function(path) {
+    _this.$__.activePaths.require(path);
+  });
+
+  return this;
+};
+
+/*!
+ * ignore
+ */
+
+Document.prototype.$__undoReset = function $__undoReset() {
+  if (this.$__.backup == null || this.$__.backup.activePaths == null) {
+    return;
+  }
+
+  this.$__.activePaths.states.modify = this.$__.backup.activePaths.modify;
+  this.$__.activePaths.states.default = this.$__.backup.activePaths.default;
+
+  this.$__.validationError = this.$__.backup.validationError;
+  this.errors = this.$__.backup.errors;
+
+  for (const dirt of this.$__dirty()) {
+    const type = dirt.value;
+
+    if (type && type[arrayAtomicsSymbol] && type[arrayAtomicsBackupSymbol]) {
+      type[arrayAtomicsSymbol] = type[arrayAtomicsBackupSymbol];
+    }
+  }
+
+  for (const subdoc of this.$__getAllSubdocs()) {
+    subdoc.$__undoReset();
+  }
+};
+
+/**
+ * Returns this documents dirty paths / vals.
+ *
+ * @api private
+ * @method $__dirty
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__dirty = function() {
+  const _this = this;
+
+  let all = this.$__.activePaths.map('modify', function(path) {
+    return {
+      path: path,
+      value: _this.$__getValue(path),
+      schema: _this.$__path(path)
+    };
+  });
+
+  // gh-2558: if we had to set a default and the value is not undefined,
+  // we have to save as well
+  all = all.concat(this.$__.activePaths.map('default', function(path) {
+    if (path === '_id' || _this.$__getValue(path) == null) {
+      return;
+    }
+    return {
+      path: path,
+      value: _this.$__getValue(path),
+      schema: _this.$__path(path)
+    };
+  }));
+
+  // Sort dirty paths in a flat hierarchy.
+  all.sort(function(a, b) {
+    return (a.path < b.path ? -1 : (a.path > b.path ? 1 : 0));
+  });
+
+  // Ignore "foo.a" if "foo" is dirty already.
+  const minimal = [];
+  let lastPath;
+  let top;
+
+  all.forEach(function(item) {
+    if (!item) {
+      return;
+    }
+    if (lastPath == null || item.path.indexOf(lastPath) !== 0) {
+      lastPath = item.path + '.';
+      minimal.push(item);
+      top = item;
+    } else if (top != null &&
+        top.value != null &&
+        top.value[arrayAtomicsSymbol] != null &&
+        top.value.hasAtomics()) {
+      // special case for top level MongooseArrays
+      // the `top` array itself and a sub path of `top` are being modified.
+      // the only way to honor all of both modifications is through a $set
+      // of entire array.
+      top.value[arrayAtomicsSymbol] = {};
+      top.value[arrayAtomicsSymbol].$set = top.value;
+    }
+  });
+
+  top = lastPath = null;
+  return minimal;
+};
+
+/**
+ * Assigns/compiles `schema` into this documents prototype.
+ *
+ * @param {Schema} schema
+ * @api private
+ * @method $__setSchema
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__setSchema = function(schema) {
+  schema.plugin(idGetter, { deduplicate: true });
+  compile(schema.tree, this, undefined, schema.options);
+
+  // Apply default getters if virtual doesn't have any (gh-6262)
+  for (const key of Object.keys(schema.virtuals)) {
+    schema.virtuals[key]._applyDefaultGetters();
+  }
+
+  this.schema = schema;
+  this[documentSchemaSymbol] = schema;
+};
+
+
+/**
+ * Get active path that were changed and are arrays
+ *
+ * @api private
+ * @method $__getArrayPathsToValidate
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__getArrayPathsToValidate = function() {
+  DocumentArray || (DocumentArray = require('./types/documentarray'));
+
+  // validate all document arrays.
+  return this.$__.activePaths
+    .map('init', 'modify', function(i) {
+      return this.$__getValue(i);
+    }.bind(this))
+    .filter(function(val) {
+      return val && val instanceof Array && val.isMongooseDocumentArray && val.length;
+    }).reduce(function(seed, array) {
+      return seed.concat(array);
+    }, [])
+    .filter(function(doc) {
+      return doc;
+    });
+};
+
+
+/**
+ * Get all subdocs (by bfs)
+ *
+ * @api private
+ * @method $__getAllSubdocs
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__getAllSubdocs = function() {
+  DocumentArray || (DocumentArray = require('./types/documentarray'));
+  Embedded = Embedded || require('./types/embedded');
+
+  function docReducer(doc, seed, path) {
+    let val = doc;
+    if (path) {
+      if (doc instanceof Document && doc[documentSchemaSymbol].paths[path]) {
+        val = doc._doc[path];
+      } else {
+        val = doc[path];
+      }
+    }
+    if (val instanceof Embedded) {
+      seed.push(val);
+    } else if (val instanceof Map) {
+      seed = Array.from(val.keys()).reduce(function(seed, path) {
+        return docReducer(val.get(path), seed, null);
+      }, seed);
+    } else if (val && val.$isSingleNested) {
+      seed = Object.keys(val._doc).reduce(function(seed, path) {
+        return docReducer(val._doc, seed, path);
+      }, seed);
+      seed.push(val);
+    } else if (val && val.isMongooseDocumentArray) {
+      val.forEach(function _docReduce(doc) {
+        if (!doc || !doc._doc) {
+          return;
+        }
+        seed = Object.keys(doc._doc).reduce(function(seed, path) {
+          return docReducer(doc._doc, seed, path);
+        }, seed);
+        if (doc instanceof Embedded) {
+          seed.push(doc);
+        }
+      });
+    } else if (val instanceof Document && val.$__isNested) {
+      seed = Object.keys(val).reduce(function(seed, path) {
+        return docReducer(val, seed, path);
+      }, seed);
+    }
+    return seed;
+  }
+
+  const _this = this;
+  const subDocs = Object.keys(this._doc).reduce(function(seed, path) {
+    return docReducer(_this, seed, path);
+  }, []);
+
+  return subDocs;
+};
+
+/*!
+ * Runs queued functions
+ */
+
+function applyQueue(doc) {
+  const q = doc.schema && doc.schema.callQueue;
+  if (!q.length) {
+    return;
+  }
+
+  for (const pair of q) {
+    if (pair[0] !== 'pre' && pair[0] !== 'post' && pair[0] !== 'on') {
+      doc[pair[0]].apply(doc, pair[1]);
+    }
+  }
+}
+
+/*!
+ * ignore
+ */
+
+Document.prototype.$__handleReject = function handleReject(err) {
+  // emit on the Model if listening
+  if (this.listeners('error').length) {
+    this.emit('error', err);
+  } else if (this.constructor.listeners && this.constructor.listeners('error').length) {
+    this.constructor.emit('error', err);
+  }
+};
+
+/**
+ * Internal helper for toObject() and toJSON() that doesn't manipulate options
+ *
+ * @api private
+ * @method $toObject
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$toObject = function(options, json) {
+  let defaultOptions = {
+    transform: true,
+    flattenDecimals: true
+  };
+
+  const path = json ? 'toJSON' : 'toObject';
+  const baseOptions = get(this, 'constructor.base.options.' + path, {});
+  const schemaOptions = get(this, 'schema.options', {});
+  // merge base default options with Schema's set default options if available.
+  // `clone` is necessary here because `utils.options` directly modifies the second input.
+  defaultOptions = utils.options(defaultOptions, clone(baseOptions));
+  defaultOptions = utils.options(defaultOptions, clone(schemaOptions[path] || {}));
+
+  // If options do not exist or is not an object, set it to empty object
+  options = utils.isPOJO(options) ? clone(options) : {};
+  options._calledWithOptions = options._calledWithOptions || clone(options);
+
+  if (!('flattenMaps' in options)) {
+    options.flattenMaps = defaultOptions.flattenMaps;
+  }
+
+  let _minimize;
+  if (options._calledWithOptions.minimize != null) {
+    _minimize = options.minimize;
+  } else if (defaultOptions.minimize != null) {
+    _minimize = defaultOptions.minimize;
+  } else {
+    _minimize = schemaOptions.minimize;
+  }
+
+  // The original options that will be passed to `clone()`. Important because
+  // `clone()` will recursively call `$toObject()` on embedded docs, so we
+  // need the original options the user passed in, plus `_isNested` and
+  // `_parentOptions` for checking whether we need to depopulate.
+  const cloneOptions = Object.assign(utils.clone(options), {
+    _isNested: true,
+    json: json,
+    minimize: _minimize
+  });
+
+  if (utils.hasUserDefinedProperty(options, 'getters')) {
+    cloneOptions.getters = options.getters;
+  }
+  if (utils.hasUserDefinedProperty(options, 'virtuals')) {
+    cloneOptions.virtuals = options.virtuals;
+  }
+
+  const depopulate = options.depopulate ||
+    get(options, '_parentOptions.depopulate', false);
+  // _isNested will only be true if this is not the top level document, we
+  // should never depopulate
+  if (depopulate && options._isNested && this.$__.wasPopulated) {
+    // populated paths that we set to a document
+    return clone(this._id, cloneOptions);
+  }
+
+  // merge default options with input options.
+  options = utils.options(defaultOptions, options);
+  options._isNested = true;
+  options.json = json;
+  options.minimize = _minimize;
+
+  cloneOptions._parentOptions = options;
+  cloneOptions._skipSingleNestedGetters = true;
+
+  const gettersOptions = Object.assign({}, cloneOptions);
+  gettersOptions._skipSingleNestedGetters = false;
+
+  // remember the root transform function
+  // to save it from being overwritten by sub-transform functions
+  const originalTransform = options.transform;
+
+  let ret = clone(this._doc, cloneOptions) || {};
+
+  if (options.getters) {
+    applyGetters(this, ret, gettersOptions);
+
+    if (options.minimize) {
+      ret = minimize(ret) || {};
+    }
+  }
+
+  if (options.virtuals || (options.getters && options.virtuals !== false)) {
+    applyVirtuals(this, ret, gettersOptions, options);
+  }
+
+  if (options.versionKey === false && this.schema.options.versionKey) {
+    delete ret[this.schema.options.versionKey];
+  }
+
+  let transform = options.transform;
+
+  // In the case where a subdocument has its own transform function, we need to
+  // check and see if the parent has a transform (options.transform) and if the
+  // child schema has a transform (this.schema.options.toObject) In this case,
+  // we need to adjust options.transform to be the child schema's transform and
+  // not the parent schema's
+  if (transform) {
+    applySchemaTypeTransforms(this, ret);
+  }
+
+  if (options.useProjection) {
+    omitDeselectedFields(this, ret);
+  }
+
+  if (transform === true || (schemaOptions.toObject && transform)) {
+    const opts = options.json ? schemaOptions.toJSON : schemaOptions.toObject;
+
+    if (opts) {
+      transform = (typeof options.transform === 'function' ? options.transform : opts.transform);
+    }
+  } else {
+    options.transform = originalTransform;
+  }
+
+  if (typeof transform === 'function') {
+    const xformed = transform(this, ret, options);
+    if (typeof xformed !== 'undefined') {
+      ret = xformed;
+    }
+  }
+
+  return ret;
+};
+
+/**
+ * Converts this document into a plain javascript object, ready for storage in MongoDB.
+ *
+ * Buffers are converted to instances of [mongodb.Binary](http://mongodb.github.com/node-mongodb-native/api-bson-generated/binary.html) for proper storage.
+ *
+ * ####Options:
+ *
+ * - `getters` apply all getters (path and virtual getters), defaults to false
+ * - `aliases` apply all aliases if `virtuals=true`, defaults to true
+ * - `virtuals` apply virtual getters (can override `getters` option), defaults to false
+ * - `minimize` remove empty objects, defaults to true
+ * - `transform` a transform function to apply to the resulting document before returning
+ * - `depopulate` depopulate any populated paths, replacing them with their original refs, defaults to false
+ * - `versionKey` whether to include the version key, defaults to true
+ * - `flattenMaps` convert Maps to POJOs. Useful if you want to JSON.stringify() the result of toObject(), defaults to false
+ * - `useProjection` set to `true` to omit fields that are excluded in this document's projection. Unless you specified a projection, this will omit any field that has `select: false` in the schema.
+ *
+ * ####Getters/Virtuals
+ *
+ * Example of only applying path getters
+ *
+ *     doc.toObject({ getters: true, virtuals: false })
+ *
+ * Example of only applying virtual getters
+ *
+ *     doc.toObject({ virtuals: true })
+ *
+ * Example of applying both path and virtual getters
+ *
+ *     doc.toObject({ getters: true })
+ *
+ * To apply these options to every document of your schema by default, set your [schemas](#schema_Schema) `toObject` option to the same argument.
+ *
+ *     schema.set('toObject', { virtuals: true })
+ *
+ * ####Transform
+ *
+ * We may need to perform a transformation of the resulting object based on some criteria, say to remove some sensitive information or return a custom object. In this case we set the optional `transform` function.
+ *
+ * Transform functions receive three arguments
+ *
+ *     function (doc, ret, options) {}
+ *
+ * - `doc` The mongoose document which is being converted
+ * - `ret` The plain object representation which has been converted
+ * - `options` The options in use (either schema options or the options passed inline)
+ *
+ * ####Example
+ *
+ *     // specify the transform schema option
+ *     if (!schema.options.toObject) schema.options.toObject = {};
+ *     schema.options.toObject.transform = function (doc, ret, options) {
+ *       // remove the _id of every document before returning the result
+ *       delete ret._id;
+ *       return ret;
+ *     }
+ *
+ *     // without the transformation in the schema
+ *     doc.toObject(); // { _id: 'anId', name: 'Wreck-it Ralph' }
+ *
+ *     // with the transformation
+ *     doc.toObject(); // { name: 'Wreck-it Ralph' }
+ *
+ * With transformations we can do a lot more than remove properties. We can even return completely new customized objects:
+ *
+ *     if (!schema.options.toObject) schema.options.toObject = {};
+ *     schema.options.toObject.transform = function (doc, ret, options) {
+ *       return { movie: ret.name }
+ *     }
+ *
+ *     // without the transformation in the schema
+ *     doc.toObject(); // { _id: 'anId', name: 'Wreck-it Ralph' }
+ *
+ *     // with the transformation
+ *     doc.toObject(); // { movie: 'Wreck-it Ralph' }
+ *
+ * _Note: if a transform function returns `undefined`, the return value will be ignored._
+ *
+ * Transformations may also be applied inline, overridding any transform set in the options:
+ *
+ *     function xform (doc, ret, options) {
+ *       return { inline: ret.name, custom: true }
+ *     }
+ *
+ *     // pass the transform as an inline option
+ *     doc.toObject({ transform: xform }); // { inline: 'Wreck-it Ralph', custom: true }
+ *
+ * If you want to skip transformations, use `transform: false`:
+ *
+ *     schema.options.toObject.hide = '_id';
+ *     schema.options.toObject.transform = function (doc, ret, options) {
+ *       if (options.hide) {
+ *         options.hide.split(' ').forEach(function (prop) {
+ *           delete ret[prop];
+ *         });
+ *       }
+ *       return ret;
+ *     }
+ *
+ *     const doc = new Doc({ _id: 'anId', secret: 47, name: 'Wreck-it Ralph' });
+ *     doc.toObject();                                        // { secret: 47, name: 'Wreck-it Ralph' }
+ *     doc.toObject({ hide: 'secret _id', transform: false });// { _id: 'anId', secret: 47, name: 'Wreck-it Ralph' }
+ *     doc.toObject({ hide: 'secret _id', transform: true }); // { name: 'Wreck-it Ralph' }
+ *
+ * If you pass a transform in `toObject()` options, Mongoose will apply the transform
+ * to [subdocuments](/docs/subdocs.html) in addition to the top-level document.
+ * Similarly, `transform: false` skips transforms for all subdocuments.
+ * Note that this is behavior is different for transforms defined in the schema:
+ * if you define a transform in `schema.options.toObject.transform`, that transform
+ * will **not** apply to subdocuments.
+ *
+ *     const memberSchema = new Schema({ name: String, email: String });
+ *     const groupSchema = new Schema({ members: [memberSchema], name: String, email });
+ *     const Group = mongoose.model('Group', groupSchema);
+ *
+ *     const doc = new Group({
+ *       name: 'Engineering',
+ *       email: 'dev@mongoosejs.io',
+ *       members: [{ name: 'Val', email: 'val@mongoosejs.io' }]
+ *     });
+ *
+ *     // Removes `email` from both top-level document **and** array elements
+ *     // { name: 'Engineering', members: [{ name: 'Val' }] }
+ *     doc.toObject({ transform: (doc, ret) => { delete ret.email; return ret; } });
+ *
+ * Transforms, like all of these options, are also available for `toJSON`. See [this guide to `JSON.stringify()`](https://thecodebarbarian.com/the-80-20-guide-to-json-stringify-in-javascript.html) to learn why `toJSON()` and `toObject()` are separate functions.
+ *
+ * See [schema options](/docs/guide.html#toObject) for some more details.
+ *
+ * _During save, no custom options are applied to the document before being sent to the database._
+ *
+ * @param {Object} [options]
+ * @param {Boolean} [options.getters=false] if true, apply all getters, including virtuals
+ * @param {Boolean} [options.virtuals=false] if true, apply virtuals, including aliases. Use `{ getters: true, virtuals: false }` to just apply getters, not virtuals
+ * @param {Boolean} [options.aliases=true] if `options.virtuals = true`, you can set `options.aliases = false` to skip applying aliases. This option is a no-op if `options.virtuals = false`.
+ * @param {Boolean} [options.minimize=true] if true, omit any empty objects from the output
+ * @param {Function|null} [options.transform=null] if set, mongoose will call this function to allow you to transform the returned object
+ * @param {Boolean} [options.depopulate=false] if true, replace any conventionally populated paths with the original id in the output. Has no affect on virtual populated paths.
+ * @param {Boolean} [options.versionKey=true] if false, exclude the version key (`__v` by default) from the output
+ * @param {Boolean} [options.flattenMaps=false] if true, convert Maps to POJOs. Useful if you want to `JSON.stringify()` the result of `toObject()`.
+ * @param {Boolean} [options.useProjection=false] - If true, omits fields that are excluded in this document's projection. Unless you specified a projection, this will omit any field that has `select: false` in the schema.
+ * @return {Object} js object
+ * @see mongodb.Binary http://mongodb.github.com/node-mongodb-native/api-bson-generated/binary.html
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.toObject = function(options) {
+  return this.$toObject(options);
+};
+
+/*!
+ * Minimizes an object, removing undefined values and empty objects
+ *
+ * @param {Object} object to minimize
+ * @return {Object}
+ */
+
+function minimize(obj) {
+  const keys = Object.keys(obj);
+  let i = keys.length;
+  let hasKeys;
+  let key;
+  let val;
+
+  while (i--) {
+    key = keys[i];
+    val = obj[key];
+
+    if (utils.isObject(val) && !Buffer.isBuffer(val)) {
+      obj[key] = minimize(val);
+    }
+
+    if (undefined === obj[key]) {
+      delete obj[key];
+      continue;
+    }
+
+    hasKeys = true;
+  }
+
+  return hasKeys
+    ? obj
+    : undefined;
+}
+
+/*!
+ * Applies virtuals properties to `json`.
+ */
+
+function applyVirtuals(self, json, options, toObjectOptions) {
+  const schema = self.schema;
+  const paths = Object.keys(schema.virtuals);
+  let i = paths.length;
+  const numPaths = i;
+  let path;
+  let assignPath;
+  let cur = self._doc;
+  let v;
+  const aliases = get(toObjectOptions, 'aliases', true);
+
+  if (!cur) {
+    return json;
+  }
+
+  options = options || {};
+  for (i = 0; i < numPaths; ++i) {
+    path = paths[i];
+
+    // Allow skipping aliases with `toObject({ virtuals: true, aliases: false })`
+    if (!aliases && schema.aliases.hasOwnProperty(path)) {
+      continue;
+    }
+
+    // We may be applying virtuals to a nested object, for example if calling
+    // `doc.nestedProp.toJSON()`. If so, the path we assign to, `assignPath`,
+    // will be a trailing substring of the `path`.
+    assignPath = path;
+    if (options.path != null) {
+      if (!path.startsWith(options.path + '.')) {
+        continue;
+      }
+      assignPath = path.substr(options.path.length + 1);
+    }
+    const parts = assignPath.split('.');
+    v = clone(self.get(path), options);
+    if (v === void 0) {
+      continue;
+    }
+    const plen = parts.length;
+    cur = json;
+    for (let j = 0; j < plen - 1; ++j) {
+      cur[parts[j]] = cur[parts[j]] || {};
+      cur = cur[parts[j]];
+    }
+    cur[parts[plen - 1]] = v;
+  }
+
+  return json;
+}
+
+/*!
+ * Applies virtuals properties to `json`.
+ *
+ * @param {Document} self
+ * @param {Object} json
+ * @return {Object} `json`
+ */
+
+function applyGetters(self, json, options) {
+  const schema = self.schema;
+  const paths = Object.keys(schema.paths);
+  let i = paths.length;
+  let path;
+  let cur = self._doc;
+  let v;
+
+  if (!cur) {
+    return json;
+  }
+
+  while (i--) {
+    path = paths[i];
+
+    const parts = path.split('.');
+    const plen = parts.length;
+    const last = plen - 1;
+    let branch = json;
+    let part;
+    cur = self._doc;
+
+    if (!self.isSelected(path)) {
+      continue;
+    }
+
+    for (let ii = 0; ii < plen; ++ii) {
+      part = parts[ii];
+      v = cur[part];
+      if (ii === last) {
+        const val = self.get(path);
+        branch[part] = clone(val, options);
+      } else if (v == null) {
+        if (part in cur) {
+          branch[part] = v;
+        }
+        break;
+      } else {
+        branch = branch[part] || (branch[part] = {});
+      }
+      cur = v;
+    }
+  }
+
+  return json;
+}
+
+/*!
+ * Applies schema type transforms to `json`.
+ *
+ * @param {Document} self
+ * @param {Object} json
+ * @return {Object} `json`
+ */
+
+function applySchemaTypeTransforms(self, json) {
+  const schema = self.schema;
+  const paths = Object.keys(schema.paths || {});
+  const cur = self._doc;
+
+  if (!cur) {
+    return json;
+  }
+
+  for (const path of paths) {
+    const schematype = schema.paths[path];
+    if (typeof schematype.options.transform === 'function') {
+      const val = self.get(path);
+      const transformedValue = schematype.options.transform.call(self, val);
+      throwErrorIfPromise(path, transformedValue);
+      json[path] = transformedValue;
+    } else if (schematype.$embeddedSchemaType != null &&
+        typeof schematype.$embeddedSchemaType.options.transform === 'function') {
+      const vals = [].concat(self.get(path));
+      const transform = schematype.$embeddedSchemaType.options.transform;
+      for (let i = 0; i < vals.length; ++i) {
+        const transformedValue = transform.call(self, vals[i]);
+        vals[i] = transformedValue;
+        throwErrorIfPromise(path, transformedValue);
+      }
+
+      json[path] = vals;
+    }
+  }
+
+  return json;
+}
+
+function throwErrorIfPromise(path, transformedValue) {
+  if (isPromise(transformedValue)) {
+    throw new Error('`transform` function must be synchronous, but the transform on path `' + path + '` returned a promise.');
+  }
+}
+
+/*!
+ * ignore
+ */
+
+function omitDeselectedFields(self, json) {
+  const schema = self.schema;
+  const paths = Object.keys(schema.paths || {});
+  const cur = self._doc;
+
+  if (!cur) {
+    return json;
+  }
+
+  let selected = self.$__.selected;
+  if (selected === void 0) {
+    selected = {};
+    queryhelpers.applyPaths(selected, schema);
+  }
+  if (selected == null || Object.keys(selected).length === 0) {
+    return json;
+  }
+
+  for (const path of paths) {
+    if (selected[path] != null && !selected[path]) {
+      delete json[path];
+    }
+  }
+
+  return json;
+}
+
+/**
+ * The return value of this method is used in calls to JSON.stringify(doc).
+ *
+ * This method accepts the same options as [Document#toObject](#document_Document-toObject). To apply the options to every document of your schema by default, set your [schemas](#schema_Schema) `toJSON` option to the same argument.
+ *
+ *     schema.set('toJSON', { virtuals: true })
+ *
+ * See [schema options](/docs/guide.html#toJSON) for details.
+ *
+ * @param {Object} options
+ * @return {Object}
+ * @see Document#toObject #document_Document-toObject
+ * @see JSON.stringify() in JavaScript https://thecodebarbarian.com/the-80-20-guide-to-json-stringify-in-javascript.html
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.toJSON = function(options) {
+  return this.$toObject(options, true);
+};
+
+/**
+ * If this document is a subdocument or populated document, returns the document's
+ * parent. Returns `undefined` otherwise.
+ *
+ * @api public
+ * @method parent
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.parent = function() {
+  return this.$__.parent;
+};
+
+/**
+ * Helper for console.log
+ *
+ * @api public
+ * @method inspect
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.inspect = function(options) {
+  const isPOJO = utils.isPOJO(options);
+  let opts;
+  if (isPOJO) {
+    opts = options;
+    opts.minimize = false;
+  }
+  const ret = this.toObject(opts);
+
+  if (ret == null) {
+    // If `toObject()` returns null, `this` is still an object, so if `inspect()`
+    // prints out null this can cause some serious confusion. See gh-7942.
+    return 'MongooseDocument { ' + ret + ' }';
+  }
+
+  return ret;
+};
+
+if (inspect.custom) {
+  /*!
+  * Avoid Node deprecation warning DEP0079
+  */
+
+  Document.prototype[inspect.custom] = Document.prototype.inspect;
+}
+
+/**
+ * Helper for console.log
+ *
+ * @api public
+ * @method toString
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.toString = function() {
+  const ret = this.inspect();
+  if (typeof ret === 'string') {
+    return ret;
+  }
+  return inspect(ret);
+};
+
+/**
+ * Returns true if the Document stores the same data as doc.
+ *
+ * Documents are considered equal when they have matching `_id`s, unless neither
+ * document has an `_id`, in which case this function falls back to using
+ * `deepEqual()`.
+ *
+ * @param {Document} doc a document to compare
+ * @return {Boolean}
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.equals = function(doc) {
+  if (!doc) {
+    return false;
+  }
+
+  const tid = this.$__getValue('_id');
+  const docid = doc.$__ != null ? doc.$__getValue('_id') : doc;
+  if (!tid && !docid) {
+    return deepEqual(this, doc);
+  }
+  return tid && tid.equals
+    ? tid.equals(docid)
+    : tid === docid;
+};
+
+/**
+ * Populates document references, executing the `callback` when complete.
+ * If you want to use promises instead, use this function with
+ * [`execPopulate()`](#document_Document-execPopulate)
+ *
+ * ####Example:
+ *
+ *     doc
+ *     .populate('company')
+ *     .populate({
+ *       path: 'notes',
+ *       match: /airline/,
+ *       select: 'text',
+ *       model: 'modelName'
+ *       options: opts
+ *     }, function (err, user) {
+ *       assert(doc._id === user._id) // the document itself is passed
+ *     })
+ *
+ *     // summary
+ *     doc.populate(path)                   // not executed
+ *     doc.populate(options);               // not executed
+ *     doc.populate(path, callback)         // executed
+ *     doc.populate(options, callback);     // executed
+ *     doc.populate(callback);              // executed
+ *     doc.populate(options).execPopulate() // executed, returns promise
+ *
+ *
+ * ####NOTE:
+ *
+ * Population does not occur unless a `callback` is passed *or* you explicitly
+ * call `execPopulate()`.
+ * Passing the same path a second time will overwrite the previous path options.
+ * See [Model.populate()](#model_Model.populate) for explaination of options.
+ *
+ * @see Model.populate #model_Model.populate
+ * @see Document.execPopulate #document_Document-execPopulate
+ * @param {String|Object} [path] The path to populate or an options object
+ * @param {Function} [callback] When passed, population is invoked
+ * @api public
+ * @return {Document} this
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.populate = function populate() {
+  if (arguments.length === 0) {
+    return this;
+  }
+
+  const pop = this.$__.populate || (this.$__.populate = {});
+  const args = utils.args(arguments);
+  let fn;
+
+  if (typeof args[args.length - 1] === 'function') {
+    fn = args.pop();
+  }
+
+  // allow `doc.populate(callback)`
+  if (args.length) {
+    // use hash to remove duplicate paths
+    const res = utils.populate.apply(null, args);
+    for (const populateOptions of res) {
+      pop[populateOptions.path] = populateOptions;
+    }
+  }
+
+  if (fn) {
+    const paths = utils.object.vals(pop);
+    this.$__.populate = undefined;
+    let topLevelModel = this.constructor;
+    if (this.$__isNested) {
+      topLevelModel = this.$__[scopeSymbol].constructor;
+      const nestedPath = this.$__.nestedPath;
+      paths.forEach(function(populateOptions) {
+        populateOptions.path = nestedPath + '.' + populateOptions.path;
+      });
+    }
+
+    // Use `$session()` by default if the document has an associated session
+    // See gh-6754
+    if (this.$session() != null) {
+      const session = this.$session();
+      paths.forEach(path => {
+        if (path.options == null) {
+          path.options = { session: session };
+          return;
+        }
+        if (!('session' in path.options)) {
+          path.options.session = session;
+        }
+      });
+    }
+
+    topLevelModel.populate(this, paths, fn);
+  }
+
+  return this;
+};
+
+/**
+ * Explicitly executes population and returns a promise. Useful for ES2015
+ * integration.
+ *
+ * ####Example:
+ *
+ *     const promise = doc.
+ *       populate('company').
+ *       populate({
+ *         path: 'notes',
+ *         match: /airline/,
+ *         select: 'text',
+ *         model: 'modelName'
+ *         options: opts
+ *       }).
+ *       execPopulate();
+ *
+ *     // summary
+ *     doc.execPopulate().then(resolve, reject);
+ *
+ *   // you can also use doc.execPopulate(options) as a shorthand for
+ *   // doc.populate(options).execPopulate()
+ *
+ *
+ * ####Example:
+ *   const promise = doc.execPopulate({ path: 'company', select: 'employees' });
+ *
+ *   // summary
+ *   promise.then(resolve,reject);
+ *
+ * @see Document.populate #document_Document-populate
+ * @api public
+ * @param {Function} [callback] optional callback. If specified, a promise will **not** be returned
+ * @return {Promise} promise that resolves to the document when population is done
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.execPopulate = function(callback) {
+  const isUsingShorthand = callback != null && typeof callback !== 'function';
+  if (isUsingShorthand) {
+    return this.populate.apply(this, arguments).execPopulate();
+  }
+
+  return promiseOrCallback(callback, cb => {
+    this.populate(cb);
+  }, this.constructor.events);
+};
+
+/**
+ * Gets _id(s) used during population of the given `path`.
+ *
+ * ####Example:
+ *
+ *     Model.findOne().populate('author').exec(function (err, doc) {
+ *       console.log(doc.author.name)         // Dr.Seuss
+ *       console.log(doc.populated('author')) // '5144cf8050f071d979c118a7'
+ *     })
+ *
+ * If the path was not populated, undefined is returned.
+ *
+ * @param {String} path
+ * @return {Array|ObjectId|Number|Buffer|String|undefined}
+ * @memberOf Document
+ * @instance
+ * @api public
+ */
+
+Document.prototype.populated = function(path, val, options) {
+  // val and options are internal
+  if (val === null || val === void 0) {
+    if (!this.$__.populated) {
+      return undefined;
+    }
+    const v = this.$__.populated[path];
+    if (v) {
+      return v.value;
+    }
+    return undefined;
+  }
+
+  // internal
+  if (val === true) {
+    if (!this.$__.populated) {
+      return undefined;
+    }
+    return this.$__.populated[path];
+  }
+
+  this.$__.populated || (this.$__.populated = {});
+  this.$__.populated[path] = { value: val, options: options };
+
+  // If this was a nested populate, make sure each populated doc knows
+  // about its populated children (gh-7685)
+  const pieces = path.split('.');
+  for (let i = 0; i < pieces.length - 1; ++i) {
+    const subpath = pieces.slice(0, i + 1).join('.');
+    const subdoc = this.get(subpath);
+    if (subdoc != null && subdoc.$__ != null && this.populated(subpath)) {
+      const rest = pieces.slice(i + 1).join('.');
+      subdoc.populated(rest, val, options);
+      // No need to continue because the above recursion should take care of
+      // marking the rest of the docs as populated
+      break;
+    }
+  }
+
+  return val;
+};
+
+/**
+ * Takes a populated field and returns it to its unpopulated state.
+ *
+ * ####Example:
+ *
+ *     Model.findOne().populate('author').exec(function (err, doc) {
+ *       console.log(doc.author.name); // Dr.Seuss
+ *       console.log(doc.depopulate('author'));
+ *       console.log(doc.author); // '5144cf8050f071d979c118a7'
+ *     })
+ *
+ * If the path was not populated, this is a no-op.
+ *
+ * @param {String} path
+ * @return {Document} this
+ * @see Document.populate #document_Document-populate
+ * @api public
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.depopulate = function(path) {
+  if (typeof path === 'string') {
+    path = path.split(' ');
+  }
+
+  let populatedIds;
+  const virtualKeys = this.$$populatedVirtuals ? Object.keys(this.$$populatedVirtuals) : [];
+  const populated = get(this, '$__.populated', {});
+
+  if (arguments.length === 0) {
+    // Depopulate all
+    for (const virtualKey of virtualKeys) {
+      delete this.$$populatedVirtuals[virtualKey];
+      delete this._doc[virtualKey];
+      delete populated[virtualKey];
+    }
+
+    const keys = Object.keys(populated);
+
+    for (const key of keys) {
+      populatedIds = this.populated(key);
+      if (!populatedIds) {
+        continue;
+      }
+      delete populated[key];
+      this.$set(key, populatedIds);
+    }
+    return this;
+  }
+
+  for (const singlePath of path) {
+    populatedIds = this.populated(singlePath);
+    delete populated[singlePath];
+
+    if (virtualKeys.indexOf(singlePath) !== -1) {
+      delete this.$$populatedVirtuals[singlePath];
+      delete this._doc[singlePath];
+    } else if (populatedIds) {
+      this.$set(singlePath, populatedIds);
+    }
+  }
+  return this;
+};
+
+
+/**
+ * Returns the full path to this document.
+ *
+ * @param {String} [path]
+ * @return {String}
+ * @api private
+ * @method $__fullPath
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.$__fullPath = function(path) {
+  // overridden in SubDocuments
+  return path || '';
+};
+
+/**
+ * Returns the changes that happened to the document
+ * in the format that will be sent to MongoDB.
+ *
+ * #### Example:
+ *
+ *     const userSchema = new Schema({
+ *       name: String,
+ *       age: Number,
+ *       country: String
+ *     });
+ *     const User = mongoose.model('User', userSchema);
+ *     const user = await User.create({
+ *       name: 'Hafez',
+ *       age: 25,
+ *       country: 'Egypt'
+ *     });
+ *
+ *     // returns an empty object, no changes happened yet
+ *     user.getChanges(); // { }
+ *
+ *     user.country = undefined;
+ *     user.age = 26;
+ *
+ *     user.getChanges(); // { $set: { age: 26 }, { $unset: { country: 1 } } }
+ *
+ *     await user.save();
+ *
+ *     user.getChanges(); // { }
+ *
+ * Modifying the object that `getChanges()` returns does not affect the document's
+ * change tracking state. Even if you `delete user.getChanges().$set`, Mongoose
+ * will still send a `$set` to the server.
+ *
+ * @return {Object}
+ * @api public
+ * @method getChanges
+ * @memberOf Document
+ * @instance
+ */
+
+Document.prototype.getChanges = function() {
+  const delta = this.$__delta();
+
+  const changes = delta ? delta[1] : {};
+  return changes;
+};
+
+/*!
+ * Module exports.
+ */
+
+Document.ValidationError = ValidationError;
+module.exports = exports = Document;