thoughts/node_modules/mongoose/lib/model.js

/*!
 * Module dependencies.
 */

var Document = require('./document')
  , MongooseArray = require('./types/array')
  , MongooseBuffer = require('./types/buffer')
  , MongooseError = require('./error')
  , VersionError = MongooseError.VersionError
  , DivergentArrayError = MongooseError.DivergentArrayError
  , Query = require('./query')
  , Schema = require('./schema')
  , Types = require('./schema/index')
  , utils = require('./utils')
  , hasOwnProperty = utils.object.hasOwnProperty
  , isMongooseObject = utils.isMongooseObject
  , EventEmitter = require('events').EventEmitter
  , merge = utils.merge
  , Promise = require('./promise')
  , assert = require('assert')
  , tick = utils.tick

var VERSION_WHERE = 1
  , VERSION_INC = 2
  , VERSION_ALL = VERSION_WHERE | VERSION_INC;

/**
 * Model constructor
 *
 * Provides the interface to MongoDB collections as well as creates document instances.
 *
 * @param {Object} doc values with which to create the document
 * @inherits Document
 * @event `error`: If listening to this event, it is emitted when a document was saved without passing a callback and an `error` occurred. If not listening, the event bubbles to the connection used to create this Model.
 * @event `index`: Emitted after `Model#ensureIndexes` completes. If an error occurred it is passed with the event.
 * @api public
 */

function Model (doc, fields, skipId) {
  Document.call(this, doc, fields, skipId);
};

/*!
 * Inherits from Document.
 *
 * All Model.prototype features are available on
 * top level (non-sub) documents.
 */

Model.prototype.__proto__ = Document.prototype;

/**
 * Connection the model uses.
 *
 * @api public
 * @property db
 */

Model.prototype.db;

/**
 * Collection the model uses.
 *
 * @api public
 * @property collection
 */

Model.prototype.collection;

/**
 * The name of the model
 *
 * @api public
 * @property modelName
 */

Model.prototype.modelName;

/*!
 * Handles doc.save() callbacks
 */

function handleSave (promise, self) {
  return tick(function handleSave (err, result) {
    if (err) {
      // If the initial insert fails provide a second chance.
      // (If we did this all the time we would break updates)
      if (self.$__.inserting) {
        self.isNew = true;
        self.emit('isNew', true);
      }
      promise.error(err);
      promise = self = null;
      return;
    }

    self.$__storeShard();

    var numAffected;
    if (result) {
      // when inserting, the array of created docs is returned
      numAffected = result.length
        ? result.length
        : result;
    } else {
      numAffected = 0;
    }

    // was this an update that required a version bump?
    if (self.$__.version && !self.$__.inserting) {
      var doIncrement = VERSION_INC === (VERSION_INC & self.$__.version);
      self.$__.version = undefined;

      // increment version if was successful
      if (numAffected > 0) {
        if (doIncrement) {
          var key = self.schema.options.versionKey;
          var version = self.getValue(key) | 0;
          self.setValue(key, version + 1);
        }
      } else {
        // the update failed. pass an error back
        promise.error(new VersionError);
        promise = self = null;
        return;
      }
    }

    self.emit('save', self, numAffected);
    promise.complete(self, numAffected);
    promise = self = null;
  });
}

/**
 * Saves this document.
 *
 * ####Example:
 *
 *     product.sold = Date.now();
 *     product.save(function (err, product, numberAffected) {
 *       if (err) ..
 *     })
 *
 * The callback will receive three parameters, `err` if an error occurred, `product` which is the saved `product`, and `numberAffected` which will be 1 when the document was found and updated in the database, otherwise 0.
 *
 * The `fn` callback is optional. If no `fn` is passed and validation fails, the validation error will be emitted on the connection used to create this model.
 *
 *     var db = mongoose.createConnection(..);
 *     var schema = new Schema(..);
 *     var Product = db.model('Product', schema);
 *
 *     db.on('error', handleError);
 *
 * However, if you desire more local error handling you can add an `error` listener to the model and handle errors there instead.
 *
 *     Product.on('error', handleError);
 *
 * @param {Function} [fn] optional callback
 * @api public
 * @see middleware http://mongoosejs.com/docs/middleware.html
 */

Model.prototype.save = function save (fn) {
  var promise = new Promise(fn)
    , complete = handleSave(promise, this)
    , options = {}

  if (this.schema.options.safe) {
    options.safe = this.schema.options.safe;
  }

  if (this.isNew) {
    // send entire doc
    var obj = this.toObject({ depopulate: 1 });
    this.$__version(true, obj);
    this.collection.insert(obj, options, complete);
    this.$__reset();
    this.isNew = false;
    this.emit('isNew', false);
    // Make it possible to retry the insert
    this.$__.inserting = true;

  } else {
    // Make sure we don't treat it as a new object on error,
    // since it already exists
    this.$__.inserting = false;

    var delta = this.$__delta();
    if (delta) {
      if (delta instanceof Error) return complete(delta);
      var where = this.$__where(delta[0]);
      this.$__reset();
      this.collection.update(where, delta[1], options, complete);
    } else {
      this.$__reset();
      complete(null);
    }

    this.emit('isNew', false);
  }
};

/*!
 * Apply the operation to the delta (update) clause as
 * well as track versioning for our where clause.
 *
 * @param {Document} self
 * @param {Object} where
 * @param {Object} delta
 * @param {Object} data
 * @param {Mixed} val
 * @param {String} [operation]
 */

function operand (self, where, delta, data, val, op) {
  // delta
  op || (op = '$set');
  if (!delta[op]) delta[op] = {};
  delta[op][data.path] = val;

  // disabled versioning?
  if (false === self.schema.options.versionKey) return;

  // already marked for versioning?
  if (VERSION_ALL === (VERSION_ALL & self.$__.version)) return;

  switch (op) {
    case '$set':
    case '$unset':
    case '$pop':
    case '$pull':
    case '$pullAll':
    case '$push':
    case '$pushAll':
    case '$addToSet':
      break;
    default:
      // nothing to do
      return;
  }

  // ensure updates sent with positional notation are
  // editing the correct array element.
  // only increment the version if an array position changes.
  // modifying elements of an array is ok if position does not change.

  if ('$push' == op || '$pushAll' == op || '$addToSet' == op) {
    self.$__.version = VERSION_INC;
  }
  else if (/^\$p/.test(op)) {
    // potentially changing array positions
    self.increment();
  }
  else if (Array.isArray(val)) {
    // $set an array
    self.increment();
  }
  // now handling $set, $unset
  else if (/\.\d+\.|\.\d+$/.test(data.path)) {
    // subpath of array
    self.$__.version = VERSION_WHERE;
  }
}

/*!
 * Compiles an update and where clause for a `val` with _atomics.
 *
 * @param {Document} self
 * @param {Object} where
 * @param {Object} delta
 * @param {Object} data
 * @param {Array} value
 */

function handleAtomics (self, where, delta, data, value) {
  if (delta.$set && delta.$set[data.path]) {
    // $set has precedence over other atomics
    return;
  }

  if ('function' == typeof value.$__getAtomics) {
    value.$__getAtomics().forEach(function (atomic) {
      var op = atomic[0];
      var val = atomic[1];
      operand(self, where, delta, data, val, op);
    })
    return;
  }

  // legacy support for plugins

  var atomics = value._atomics
    , ops = Object.keys(atomics)
    , i = ops.length
    , val
    , op;

  if (0 === i) {
    // $set

    if (isMongooseObject(value)) {
      value = value.toObject({ depopulate: 1 });
    } else if (value.valueOf) {
      value = value.valueOf();
    }

    return operand(self, where, delta, data, value);
  }

  while (i--) {
    op = ops[i];
    val = atomics[op];

    if (isMongooseObject(val)) {
      val = val.toObject({ depopulate: 1 })
    } else if (Array.isArray(val)) {
      val = val.map(function (mem) {
        return isMongooseObject(mem)
          ? mem.toObject({ depopulate: 1 })
          : mem;
      })
    } else if (val.valueOf) {
      val = val.valueOf()
    }

    if ('$addToSet' === op)
      val = { $each: val };

    operand(self, where, delta, data, val, op);
  }
}

/**
 * Produces a special query document of the modified properties used in updates.
 *
 * @api private
 * @method $__delta
 * @memberOf Model
 */

Model.prototype.$__delta = function () {
  var dirty = this.$__dirty();
  if (!dirty.length && VERSION_ALL != this.$__.version) return;

  var where = {}
    , delta = {}
    , len = dirty.length
    , divergent = []
    , d = 0
    , val
    , obj

  for (; d < len; ++d) {
    var data = dirty[d]
    var value = data.value
    var schema = data.schema

    var match = checkDivergentArray(this, data.path, value);
    if (match) {
      divergent.push(match);
      continue;
    }

    if (divergent.length) continue;

    if (undefined === value) {
      operand(this, where, delta, data, 1, '$unset');

    } else if (null === value) {
      operand(this, where, delta, data, null);

    } else if (value._path && value._atomics) {
      // arrays and other custom types (support plugins etc)
      handleAtomics(this, where, delta, data, value);

    } else if (value._path && Buffer.isBuffer(value)) {
      // MongooseBuffer
      value = value.toObject();
      operand(this, where, delta, data, value);

    } else {
      value = utils.clone(value, { depopulate: 1 });
      operand(this, where, delta, data, value);
    }
  }

  if (divergent.length) {
    return new DivergentArrayError(divergent);
  }

  if (this.$__.version) {
    this.$__version(where, delta);
  }

  return [where, delta];
}

/*!
 * Determine if array was populated with some form of filter and is now
 * being updated in a manner which could overwrite data unintentionally.
 *
 * @see https://github.com/LearnBoost/mongoose/issues/1334
 * @param {Document} doc
 * @param {String} path
 * @return {String|undefined}
 */

function checkDivergentArray (doc, path, array) {
  // see if we populated this path
  var pop = doc.populated(path, true);

  if (!pop && doc.$__.selected) {
    // If any array was selected using an $elemMatch projection, we deny the update.
    // NOTE: MongoDB only supports projected $elemMatch on top level array.
    var top = path.split('.')[0];
    if (doc.$__.selected[top] && doc.$__.selected[top].$elemMatch) {
      return top;
    }
  }

  if (!(pop && array instanceof MongooseArray)) return;

  // If the array was populated using options that prevented all
  // documents from being returned (match, skip, limit) or they
  // deselected the _id field, $pop and $set of the array are
  // not safe operations. If _id was deselected, we do not know
  // how to remove elements. $pop will pop off the _id from the end
  // of the array in the db which is not guaranteed to be the
  // same as the last element we have here. $set of the entire array
  // would be similarily destructive as we never received all
  // elements of the array and potentially would overwrite data.
  var check = pop.options.match ||
              pop.options.options && hasOwnProperty(pop.options.options, 'limit') || // 0 is not permitted
              pop.options.options && pop.options.options.skip || // 0 is permitted
              pop.options.select && // deselected _id?
                (0 === pop.options.select._id ||
                /\s?-_id\s?/.test(pop.options.select))

  if (check) {
    var atomics = array._atomics;
    if (0 === Object.keys(atomics).length || atomics.$set || atomics.$pop) {
      return path;
    }
  }
}

/**
 * Appends versioning to the where and update clauses.
 *
 * @api private
 * @method $__version
 * @memberOf Model
 */

Model.prototype.$__version = function (where, delta) {
  var key = this.schema.options.versionKey;

  if (true === where) {
    // this is an insert
    if (key) this.setValue(key, delta[key] = 0);
    return;
  }

  // updates

  // only apply versioning if our versionKey was selected. else
  // there is no way to select the correct version. we could fail
  // fast here and force them to include the versionKey but
  // thats a bit intrusive. can we do this automatically?
  if (!this.isSelected(key)) {
    return;
  }

  // $push $addToSet don't need the where clause set
  if (VERSION_WHERE === (VERSION_WHERE & this.$__.version)) {
    where[key] = this.getValue(key);
  }

  if (VERSION_INC === (VERSION_INC & this.$__.version)) {
    delta.$inc || (delta.$inc = {});
    delta.$inc[key] = 1;
  }
}

/**
 * Signal that we desire an increment of this documents version.
 *
 * ####Example:
 *
 *     Model.findById(id, function (err, doc) {
 *       doc.increment();
 *       doc.save(function (err) { .. })
 *     })
 *
 * @see versionKeys http://mongoosejs.com/docs/guide.html#versionKey
 * @api public
 */

Model.prototype.increment = function increment () {
  this.$__.version = VERSION_ALL;
  return this;
}

/**
 * Returns a query object which applies shardkeys if they exist.
 *
 * @api private
 * @method $__where
 * @memberOf Model
 */

Model.prototype.$__where = function _where (where) {
  where || (where = {});

  var paths
    , len

  if (this.$__.shardval) {
    paths = Object.keys(this.$__.shardval)
    len = paths.length

    for (var i = 0; i < len; ++i) {
      where[paths[i]] = this.$__.shardval[paths[i]];
    }
  }

  where._id = this._doc._id;
  return where;
}

/**
 * Removes this document from the db.
 *
 * ####Example:
 *
 *     product.remove(function (err) {
 *       if (err) return handleError(err);
 *       Product.findById(product._id, function (err, product) {
 *         console.log(product) // null
 *       })
 *     })
 *
 * @param {Function} [fn] optional callback
 * @api public
 */

Model.prototype.remove = function remove (fn) {
  if (this.$__.removing) {
    this.$__.removing.addBack(fn);
    return this;
  }

  var promise = this.$__.removing = new Promise(fn)
    , where = this.$__where()
    , self = this
    , options = {}

  if (this.schema.options.safe) {
    options.safe = this.schema.options.safe;
  }

  this.collection.remove(where, options, tick(function (err) {
    if (err) {
      promise.error(err);
      promise = self = self.$__.removing = where = options = null;
      return;
    }
    self.emit('remove', self);
    promise.complete();
    promise = self = where = options = null;
  }));

  return this;
};

/**
 * Returns another Model instance.
 *
 * ####Example:
 *
 *     var doc = new Tank;
 *     doc.model('User').findById(id, callback);
 *
 * @param {String} name model name
 * @api public
 */

Model.prototype.model = function model (name) {
  return this.db.model(name);
};

// Model (class) features

/*!
 * Give the constructor the ability to emit events.
 */

for (var i in EventEmitter.prototype)
  Model[i] = EventEmitter.prototype[i];

/**
 * Called when the model compiles.
 *
 * @api private
 */

Model.init = function init () {
  if (this.schema.options.autoIndex) {
    this.ensureIndexes();
  }

  this.schema.emit('init', this);
};

/**
 * Sends `ensureIndex` commands to mongo for each index declared in the schema.
 *
 * ####Example:
 *
 *     Event.ensureIndexes(function (err) {
 *       if (err) return handleError(err);
 *     });
 *
 * After completion, an `index` event is emitted on this `Model` passing an error if one occurred.
 *
 * ####Example:
 *
 *     var eventSchema = new Schema({ thing: { type: 'string', unique: true }})
 *     var Event = mongoose.model('Event', eventSchema);
 *
 *     Event.on('index', function (err) {
 *       if (err) console.error(err); // error occurred during index creation
 *     })
 *
 * _NOTE: It is not recommended that you run this in production. Index creation may impact database performance depending on your load. Use with caution._
 *
 * The `ensureIndex` commands are not sent in parallel. This is to avoid the `MongoError: cannot add index with a background operation in progress` error. See [this ticket](https://github.com/LearnBoost/mongoose/issues/1365) for more information.
 *
 * @param {Function} [cb] optional callback
 * @api public
 */

Model.ensureIndexes = function ensureIndexes (cb) {
  var indexes = this.schema.indexes();
  if (!indexes.length) {
    return cb && process.nextTick(cb);
  }

  // Indexes are created one-by-one to support how MongoDB < 2.4 deals
  // with background indexes.

  var self = this
    , safe = self.schema.options.safe

  function done (err) {
    self.emit('index', err);
    cb && cb(err);
  }

  function create () {
    var index = indexes.shift();
    if (!index) return done();

    var options = index[1];
    options.safe = safe;
    self.collection.ensureIndex(index[0], options, tick(function (err) {
      if (err) return done(err);
      create();
    }));
  }

  create();
}

/**
 * Schema the model uses.
 *
 * @property schema
 * @receiver Model
 * @api public
 */

Model.schema;

/*!
 * Connection instance the model uses.
 *
 * @property db
 * @receiver Model
 * @api public
 */

Model.db;

/*!
 * Collection the model uses.
 *
 * @property collection
 * @receiver Model
 * @api public
 */

Model.collection;

/**
 * Base Mongoose instance the model uses.
 *
 * @property base
 * @receiver Model
 * @api public
 */

Model.base;

/**
 * Removes documents from the collection.
 *
 * ####Example:
 *
 *     Comment.remove({ title: 'baby born from alien father' }, function (err) {
 *
 *     });
 *
 * ####Note:
 *
 * To remove documents without waiting for a response from MongoDB, do not pass a `callback`, then call `exec` on the returned [Query](#query-js):
 *
 *     var query = Comment.remove({ _id: id });
 *     query.exec();
 *
 * ####Note:
 *
 * This method sends a remove command directly to MongoDB, no Mongoose documents are involved. Because no Mongoose documents are involved, _no middleware (hooks) are executed_.
 *
 * @param {Object} conditions
 * @param {Function} [callback]
 * @return {Query}
 * @api public
 */

Model.remove = function remove (conditions, callback) {
  if ('function' === typeof conditions) {
    callback = conditions;
    conditions = {};
  }

  var query = new Query(conditions).bind(this, 'remove');

  if ('undefined' === typeof callback)
    return query;

  this._applyNamedScope(query);
  return query.remove(callback);
};

/**
 * Finds documents
 *
 * The `conditions` are cast to their respective SchemaTypes before the command is sent.
 *
 * ####Examples:
 *
 *     // named john and at least 18
 *     MyModel.find({ name: 'john', age: { $gte: 18 }});
 *
 *     // executes immediately, passing results to callback
 *     MyModel.find({ name: 'john', age: { $gte: 18 }}, function (err, docs) {});
 *
 *     // name LIKE john and only selecting the "name" and "friends" fields, executing immediately
 *     MyModel.find({ name: /john/i }, 'name friends', function (err, docs) { })
 *
 *     // passing options
 *     MyModel.find({ name: /john/i }, null, { skip: 10 })
 *
 *     // passing options and executing immediately
 *     MyModel.find({ name: /john/i }, null, { skip: 10 }, function (err, docs) {});
 *
 *     // executing a query explicitly
 *     var query = MyModel.find({ name: /john/i }, null, { skip: 10 })
 *     query.exec(function (err, docs) {});
 *
 *     // using the promise returned from executing a query
 *     var query = MyModel.find({ name: /john/i }, null, { skip: 10 });
 *     var promise = query.exec();
 *     promise.addBack(function (err, docs) {});
 *
 * @param {Object} conditions
 * @param {Object} [fields] optional fields to select
 * @param {Object} [options] optional
 * @param {Function} [callback]
 * @return {Query}
 * @see field selection #query_Query-select
 * @see promise #promise-js
 * @api public
 */

Model.find = function find (conditions, fields, options, callback) {
  if ('function' == typeof conditions) {
    callback = conditions;
    conditions = {};
    fields = null;
    options = null;
  } else if ('function' == typeof fields) {
    callback = fields;
    fields = null;
    options = null;
  } else if ('function' == typeof options) {
    callback = options;
    options = null;
  }

  var query = new Query(conditions, options);
  query.bind(this, 'find');
  query.select(fields);

  if ('undefined' === typeof callback)
    return query;

  this._applyNamedScope(query);
  return query.find(callback);
};

/**
 * Merges the current named scope query into `query`.
 *
 * @param {Query} query
 * @return {Query}
 * @api private
 */

Model._applyNamedScope = function _applyNamedScope (query) {
  var cQuery = this._cumulativeQuery;

  if (cQuery) {
    merge(query._conditions, cQuery._conditions);
    if (query._fields && cQuery._fields)
      merge(query._fields, cQuery._fields);
    if (query.options && cQuery.options)
      merge(query.options, cQuery.options);
    delete this._cumulativeQuery;
  }

  return query;
}

/**
 * Finds a single document by id.
 *
 * The `id` is cast based on the Schema before sending the command.
 *
 * ####Example:
 *
 *     // find adventure by id and execute immediately
 *     Adventure.findById(id, function (err, adventure) {});
 *
 *     // same as above
 *     Adventure.findById(id).exec(callback);
 *
 *     // select only the adventures name and length
 *     Adventure.findById(id, 'name length', function (err, adventure) {});
 *
 *     // same as above
 *     Adventure.findById(id, 'name length').exec(callback);
 *
 *     // include all properties except for `length`
 *     Adventure.findById(id, '-length').exec(function (err, adventure) {});
 *
 *     // passing options (in this case return the raw js objects, not mongoose documents by passing `lean`
 *     Adventure.findById(id, 'name', { lean: true }, function (err, doc) {});
 *
 *     // same as above
 *     Adventure.findById(id, 'name').lean().exec(function (err, doc) {});
 *
 * @param {ObjectId|HexId} id objectid, or a value that can be casted to one
 * @param {Object} [fields] optional fields to select
 * @param {Object} [options] optional
 * @param {Function} [callback]
 * @return {Query}
 * @see field selection #query_Query-select
 * @see lean queries #query_Query-lean
 * @api public
 */

Model.findById = function findById (id, fields, options, callback) {
  return this.findOne({ _id: id }, fields, options, callback);
};

/**
 * Finds one document.
 *
 * The `conditions` are cast to their respective SchemaTypes before the command is sent.
 *
 * ####Example:
 *
 *     // find one iphone adventures - iphone adventures??
 *     Adventure.findOne({ type: 'iphone' }, function (err, adventure) {});
 *
 *     // same as above
 *     Adventure.findOne({ type: 'iphone' }).exec(function (err, adventure) {});
 *
 *     // select only the adventures name
 *     Adventure.findOne({ type: 'iphone' }, 'name', function (err, adventure) {});
 *
 *     // same as above
 *     Adventure.findOne({ type: 'iphone' }, 'name').exec(function (err, adventure) {});
 *
 *     // specify options, in this case lean
 *     Adventure.findOne({ type: 'iphone' }, 'name', { lean: true }, callback);
 *
 *     // same as above
 *     Adventure.findOne({ type: 'iphone' }, 'name', { lean: true }).exec(callback);
 *
 *     // chaining findOne queries (same as above)
 *     Adventure.findOne({ type: 'iphone' }).select('name').lean().exec(callback);
 *
 * @param {Object} conditions
 * @param {Object} [fields] optional fields to select
 * @param {Object} [options] optional
 * @param {Function} [callback]
 * @return {Query}
 * @see field selection #query_Query-select
 * @see lean queries #query_Query-lean
 * @api public
 */

Model.findOne = function findOne (conditions, fields, options, callback) {
  if ('function' == typeof options) {
    callback = options;
    options = null;
  } else if ('function' == typeof fields) {
    callback = fields;
    fields = null;
    options = null;
  } else if ('function' == typeof conditions) {
    callback = conditions;
    conditions = {};
    fields = null;
    options = null;
  }

  var query = new Query(conditions, options).select(fields).bind(this, 'findOne');

  if ('undefined' == typeof callback)
    return query;

  this._applyNamedScope(query);
  return query.findOne(callback);
};

/**
 * Counts number of matching documents in a database collection.
 *
 * ####Example:
 *
 *     Adventure.count({ type: 'jungle' }, function (err, count) {
 *       if (err) ..
 *       console.log('there are %d jungle adventures', count);
 *     });
 *
 * @param {Object} conditions
 * @param {Function} [callback]
 * @return {Query}
 * @api public
 */

Model.count = function count (conditions, callback) {
  if ('function' === typeof conditions)
    callback = conditions, conditions = {};

  var query = new Query(conditions).bind(this, 'count');
  if ('undefined' == typeof callback)
    return query;

  this._applyNamedScope(query);
  return query.count(callback);
};

/**
 * Executes a DISTINCT command
 *
 * ####Example
 *
 *     Link.distinct('url', { clicks: {$gt: 100}}, function (err, result) {
 *       if (err) return handleError(err);
 *
 *       assert(Array.isArray(result));
 *       console.log('unique urls with more than 100 clicks', result);
 *     })
 *
 * @param {String} field
 * @param {Object} [conditions] optional
 * @param {Function} [callback]
 * @return {Query}
 * @api public
 */

Model.distinct = function distinct (field, conditions, callback) {
  if ('function' == typeof conditions) {
    callback = conditions;
    conditions = {};
  }

  var query = new Query(conditions).bind(this, 'distinct');
  if ('undefined' == typeof callback) {
    query._distinctArg = field;
    return query;
  }

  this._applyNamedScope(query);
  return query.distinct(field, callback);
};

/**
 * Creates a Query, applies the passed conditions, and returns the Query.
 *
 * For example, instead of writing:
 *
 *     User.find({age: {$gte: 21, $lte: 65}}, callback);
 *
 * we can instead write:
 *
 *     User.where('age').gte(21).lte(65).exec(callback);
 *
 * Since the Query class also supports `where` you can continue chaining
 *
 *     User
 *     .where('age').gte(21).lte(65)
 *     .where('name', /^b/i)
 *     ... etc
 *
 * @param {String} path
 * @param {Object} [val] optional value
 * @return {Query}
 * @api public
 */

Model.where = function where (path, val) {
  var q = new Query().bind(this, 'find');
  return q.where.apply(q, arguments);
};

/**
 * Creates a `Query` and specifies a `$where` condition.
 *
 * Sometimes you need to query for things in mongodb using a JavaScript expression. You can do so via `find({ $where: javascript })`, or you can use the mongoose shortcut method $where via a Query chain or from your mongoose Model.
 *
 *     Blog.$where('this.comments.length > 5').exec(function (err, docs) {});
 *
 * @param {String|Function} argument is a javascript string or anonymous function
 * @method $where
 * @memberOf Model
 * @return {Query}
 * @see Query.$where #query_Query-%24where
 * @api public
 */

Model.$where = function $where () {
  var q = new Query().bind(this, 'find');
  return q.$where.apply(q, arguments);
};

/**
 * Issues a mongodb findAndModify update command.
 *
 * Finds a matching document, updates it according to the `update` arg, passing any `options`, and returns the found document (if any) to the callback. The query executes immediately if `callback` is passed else a Query object is returned.
 *
 * ####Options:
 *
 * - `new`: bool - true to return the modified document rather than the original. defaults to true
 * - `upsert`: bool - creates the object if it doesn't exist. defaults to false.
 * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
 * - `select`: sets the document fields to return
 *
 * ####Examples:
 *
 *     A.findOneAndUpdate(conditions, update, options, callback) // executes
 *     A.findOneAndUpdate(conditions, update, options)  // returns Query
 *     A.findOneAndUpdate(conditions, update, callback) // executes
 *     A.findOneAndUpdate(conditions, update)           // returns Query
 *     A.findOneAndUpdate()                             // returns Query
 *
 * ####Note:
 *
 * All top level update keys which are not `atomic` operation names are treated as set operations:
 *
 * ####Example:
 *
 *     var query = { name: 'borne' };
 *     Model.findOneAndUpdate(query, { name: 'jason borne' }, options, callback)
 *
 *     // is sent as
 *     Model.findOneAndUpdate(query, { $set: { name: 'jason borne' }}, options, callback)
 *
 * This helps prevent accidentally overwriting your document with `{ name: 'jason borne' }`.
 *
 * ####Note:
 *
 * Although values are cast to their appropriate types when using the findAndModify helpers, the following are *not* applied:
 *
 * - defaults
 * - setters
 * - validators
 * - middleware
 *
 * If you need those features, use the traditional approach of first retrieving the document.
 *
 *     Model.findOne({ name: 'borne' }, function (err, doc) {
 *       if (err) ..
 *       doc.name = 'jason borne';
 *       doc.save(callback);
 *     })
 *
 * @param {Object} [conditions]
 * @param {Object} [update]
 * @param {Object} [options]
 * @param {Function} [callback]
 * @return {Query}
 * @see mongodb http://www.mongodb.org/display/DOCS/findAndModify+Command
 * @api public
 */

Model.findOneAndUpdate = function (conditions, update, options, callback) {
  if ('function' == typeof options) {
    callback = options;
    options = null;
  }
  else if (1 === arguments.length) {
    if ('function' == typeof conditions) {
      var msg = 'Model.findOneAndUpdate(): First argument must not be a function.\n\n'
              + '  ' + this.modelName + '.findOneAndUpdate(conditions, update, options, callback)\n'
              + '  ' + this.modelName + '.findOneAndUpdate(conditions, update, options)\n'
              + '  ' + this.modelName + '.findOneAndUpdate(conditions, update)\n'
              + '  ' + this.modelName + '.findOneAndUpdate(update)\n'
              + '  ' + this.modelName + '.findOneAndUpdate()\n';
      throw new TypeError(msg)
    }
    update = conditions;
    conditions = undefined;
  }

  var fields;
  if (options && options.fields) {
    fields = options.fields;
    options.fields = undefined;
  }

  var query = new Query(conditions);
  query.setOptions(options);
  query.select(fields);
  query.bind(this, 'findOneAndUpdate', update);

  if ('undefined' == typeof callback)
    return query;

  this._applyNamedScope(query);
  return query.findOneAndUpdate(callback);
}

/**
 * Issues a mongodb findAndModify update command by a documents id.
 *
 * Finds a matching document, updates it according to the `update` arg, passing any `options`, and returns the found document (if any) to the callback. The query executes immediately if `callback` is passed else a Query object is returned.
 *
 * ####Options:
 *
 * - `new`: bool - true to return the modified document rather than the original. defaults to true
 * - `upsert`: bool - creates the object if it doesn't exist. defaults to false.
 * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
 * - `select`: sets the document fields to return
 *
 * ####Examples:
 *
 *     A.findByIdAndUpdate(id, update, options, callback) // executes
 *     A.findByIdAndUpdate(id, update, options)  // returns Query
 *     A.findByIdAndUpdate(id, update, callback) // executes
 *     A.findByIdAndUpdate(id, update)           // returns Query
 *     A.findByIdAndUpdate()                     // returns Query
 *
 * Finds a matching document, updates it according to the `update` arg, passing any `options`, and returns the found document (if any) to the callback. The query executes      immediately if `callback` is passed else a Query object is returned.
 *
 * ####Options:
 *
 * - `new`: bool - true to return the modified document rather than the original. defaults to true
 * - `upsert`: bool - creates the object if it doesn't exist. defaults to false.
 * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
 *
 * ####Note:
 *
 * All top level update keys which are not `atomic` operation names are treated as set operations:
 *
 * ####Example:
 *
 *     Model.findByIdAndUpdate(id, { name: 'jason borne' }, options, callback)
 *
 *     // is sent as
 *     Model.findByIdAndUpdate(id, { $set: { name: 'jason borne' }}, options, callback)
 *
 * This helps prevent accidentally overwriting your document with `{ name: 'jason borne' }`.
 *
 * ####Note:
 *
 * Although values are cast to their appropriate types when using the findAndModify helpers, the following are *not* applied:
 *
 * - defaults
 * - setters
 * - validators
 * - middleware
 *
 * If you need those features, use the traditional approach of first retrieving the document.
 *
 *     Model.findById(id, function (err, doc) {
 *       if (err) ..
 *       doc.name = 'jason borne';
 *       doc.save(callback);
 *     })
 *
 * @param {ObjectId|HexId} id an ObjectId or string that can be cast to one.
 * @param {Object} [update]
 * @param {Object} [options]
 * @param {Function} [callback]
 * @return {Query}
 * @see Model.findOneAndUpdate #model_Model.findOneAndUpdate
 * @see mongodb http://www.mongodb.org/display/DOCS/findAndModify+Command
 * @api public
 */

Model.findByIdAndUpdate = function (id, update, options, callback) {
  var args;

  if (1 === arguments.length) {
    if ('function' == typeof id) {
      var msg = 'Model.findByIdAndUpdate(): First argument must not be a function.\n\n'
                + '  ' + this.modelName + '.findByIdAndUpdate(id, callback)\n'
                + '  ' + this.modelName + '.findByIdAndUpdate(id)\n'
                + '  ' + this.modelName + '.findByIdAndUpdate()\n';
      throw new TypeError(msg)
    }
    return this.findOneAndUpdate({_id: id }, undefined);
  }

  args = utils.args(arguments, 1);
  args.unshift({ _id: id });
  return this.findOneAndUpdate.apply(this, args);
}

/**
 * Issue a mongodb findAndModify remove command.
 *
 * Finds a matching document, removes it, passing the found document (if any) to the callback.
 *
 * Executes immediately if `callback` is passed else a Query object is returned.
 *
 * ####Options:
 *
 * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
 * - `select`: sets the document fields to return
 *
 * ####Examples:
 *
 *     A.findOneAndRemove(conditions, options, callback) // executes
 *     A.findOneAndRemove(conditions, options)  // return Query
 *     A.findOneAndRemove(conditions, callback) // executes
 *     A.findOneAndRemove(conditions) // returns Query
 *     A.findOneAndRemove()           // returns Query
 *
 * Although values are cast to their appropriate types when using the findAndModify helpers, the following are *not* applied:
 *
 * - defaults
 * - setters
 * - validators
 * - middleware
 *
 * If you need those features, use the traditional approach of first retrieving the document.
 *
 *     Model.findById(id, function (err, doc) {
 *       if (err) ..
 *       doc.remove(callback);
 *     })
 *
 * @param {Object} conditions
 * @param {Object} [options]
 * @param {Function} [callback]
 * @return {Query}
 * @see mongodb http://www.mongodb.org/display/DOCS/findAndModify+Command
 * @api public
 */

Model.findOneAndRemove = function (conditions, options, callback) {
  if (1 === arguments.length && 'function' == typeof conditions) {
    var msg = 'Model.findOneAndRemove(): First argument must not be a function.\n\n'
              + '  ' + this.modelName + '.findOneAndRemove(conditions, callback)\n'
              + '  ' + this.modelName + '.findOneAndRemove(conditions)\n'
              + '  ' + this.modelName + '.findOneAndRemove()\n';
    throw new TypeError(msg)
  }

  if ('function' == typeof options) {
    callback = options;
    options = undefined;
  }

  var fields;
  if (options) {
    fields = options.select;
    options.select = undefined;
  }

  var query = new Query(conditions);
  query.setOptions(options);
  query.select(fields);
  query.bind(this, 'findOneAndRemove');

  if ('undefined' == typeof callback)
    return query;

  this._applyNamedScope(query);
  return query.findOneAndRemove(callback);
}

/**
 * Issue a mongodb findAndModify remove command by a documents id.
 *
 * Finds a matching document, removes it, passing the found document (if any) to the callback.
 *
 * Executes immediately if `callback` is passed, else a `Query` object is returned.
 *
 * ####Options:
 *
 * - `sort`: if multiple docs are found by the conditions, sets the sort order to choose which doc to update
 * - `select`: sets the document fields to return
 *
 * ####Examples:
 *
 *     A.findByIdAndRemove(id, options, callback) // executes
 *     A.findByIdAndRemove(id, options)  // return Query
 *     A.findByIdAndRemove(id, callback) // executes
 *     A.findByIdAndRemove(id) // returns Query
 *     A.findByIdAndRemove()           // returns Query
 *
 * @param {ObjectId|HexString} id ObjectId or string that can be cast to one
 * @param {Object} [options]
 * @param {Function} [callback]
 * @return {Query}
 * @see Model.findOneAndRemove #model_Model.findOneAndRemove
 * @see mongodb http://www.mongodb.org/display/DOCS/findAndModify+Command
 */

Model.findByIdAndRemove = function (id, options, callback) {
  if (1 === arguments.length && 'function' == typeof id) {
    var msg = 'Model.findByIdAndRemove(): First argument must not be a function.\n\n'
              + '  ' + this.modelName + '.findByIdAndRemove(id, callback)\n'
              + '  ' + this.modelName + '.findByIdAndRemove(id)\n'
              + '  ' + this.modelName + '.findByIdAndRemove()\n';
    throw new TypeError(msg)
  }

  return this.findOneAndRemove({ _id: id }, options, callback);
}

/**
 * Shortcut for creating a new Document that is automatically saved to the db if valid.
 *
 * ####Example:
 *
 *     Candy.create({ type: 'jelly bean' }, { type: 'snickers' }, function (err, jellybean, snickers) {
 *       if (err) // ...
 *     });
 *
 *     var array = [{ type: 'jelly bean' }, { type: 'snickers' }];
 *     Candy.create(array, function (err, jellybean, snickers) {
 *       if (err) // ...
 *     });
 *
 * @param {Array|Object...} doc
 * @param {Function} fn callback
 * @api public
 */

Model.create = function create (doc, fn) {
  if (1 === arguments.length) {
    return 'function' === typeof doc && doc(null);
  }

  var self = this
    , docs = [null]
    , promise
    , count
    , args

  if (Array.isArray(doc)) {
    args = doc;
  } else {
    args = utils.args(arguments, 0, arguments.length - 1);
    fn = arguments[arguments.length - 1];
  }

  if (0 === args.length) return fn(null);

  promise = new Promise(fn);
  count = args.length;

  args.forEach(function (arg, i) {
    var doc = new self(arg);
    docs[i+1] = doc;
    doc.save(function (err) {
      if (err) return promise.error(err);
      --count || fn.apply(null, docs);
    });
  });
};

/**
 * Updates documents in the database without returning them.
 *
 * ####Examples:
 *
 *     MyModel.update({ age: { $gt: 18 } }, { oldEnough: true }, fn);
 *     MyModel.update({ name: 'Tobi' }, { ferret: true }, { multi: true }, function (err, numberAffected, raw) {
 *       if (err) return handleError(err);
 *       console.log('The number of updated documents was %d', numberAffected);
 *       console.log('The raw response from Mongo was ', raw);
 *     });
 *
 * ####Valid options:
 *
 *  - `safe` (boolean) safe mode (defaults to value set in schema (true))
 *  - `upsert` (boolean) whether to create the doc if it doesn't match (false)
 *  - `multi` (boolean) whether multiple documents should be updated (false)
 *  - `strict` (boolean) overrides the `strict` option for this update
 *
 * All `update` values are cast to their appropriate SchemaTypes before being sent.
 *
 * The `callback` function receives `(err, numberAffected, rawResponse)`.
 *
 * - `err` is the error if any occurred
 * - `numberAffected` is the count of updated documents Mongo reported
 * - `rawResponse` is the full response from Mongo
 *
 * ####Note:
 *
 * All top level keys which are not `atomic` operation names are treated as set operations:
 *
 * ####Example:
 *
 *     var query = { name: 'borne' };
 *     Model.update(query, { name: 'jason borne' }, options, callback)
 *
 *     // is sent as
 *     Model.update(query, { $set: { name: 'jason borne' }}, options, callback)
 *
 * This helps prevent accidentally overwriting all documents in your collection with `{ name: 'jason borne' }`.
 *
 * ####Note:
 *
 * Be careful to not use an existing model instance for the update clause (this won't work and can cause weird behavior like infinite loops). Also, ensure that the update clause does not have an _id property, which causes Mongo to return a "Mod on _id not allowed" error.
 *
 * ####Note:
 *
 * To update documents without waiting for a response from MongoDB, do not pass a `callback`, then call `exec` on the returned [Query](#query-js):
 *
 *     Comment.update({ _id: id }, { $set: { text: 'changed' }}).exec();
 *
 * ####Note:
 *
 * Although values are casted to their appropriate types when using update, the following are *not* applied:
 *
 * - defaults
 * - setters
 * - validators
 * - middleware
 *
 * If you need those features, use the traditional approach of first retrieving the document.
 *
 *     Model.findOne({ name: 'borne' }, function (err, doc) {
 *       if (err) ..
 *       doc.name = 'jason borne';
 *       doc.save(callback);
 *     })
 *
 * @see strict schemas http://mongoosejs.com/docs/guide.html#strict
 * @param {Object} conditions
 * @param {Object} update
 * @param {Object} [options]
 * @param {Function} [callback]
 * @return {Query}
 * @api public
 */

Model.update = function update (conditions, doc, options, callback) {
  if (arguments.length < 4) {
    if ('function' === typeof options) {
      // Scenario: update(conditions, doc, callback)
      callback = options;
      options = null;
    } else if ('function' === typeof doc) {
      // Scenario: update(doc, callback);
      callback = doc;
      doc = conditions;
      conditions = {};
      options = null;
    }
  }

  var query = new Query(conditions, options).bind(this, 'update', doc);

  if ('undefined' == typeof callback)
    return query;

  this._applyNamedScope(query);
  return query.update(doc, callback);
};

/**
 * Executes a mapReduce command.
 *
 * `o` is an object specifying all mapReduce options as well as the map and reduce functions. All options are delegated to the driver implementation.
 *
 * ####Example:
 *
 *     var o = {};
 *     o.map = function () { emit(this.name, 1) }
 *     o.reduce = function (k, vals) { return vals.length }
 *     User.mapReduce(o, function (err, results) {
 *       console.log(results)
 *     })
 *
 * ####Other options:
 *
 * - `query` {Object} query filter object.
 * - `limit` {Number} max number of documents
 * - `keeptemp` {Boolean, default:false} keep temporary data
 * - `finalize` {Function} finalize function
 * - `scope` {Object} scope variables exposed to map/reduce/finalize during execution
 * - `jsMode` {Boolean, default:false} it is possible to make the execution stay in JS. Provided in MongoDB > 2.0.X
 * - `verbose` {Boolean, default:false} provide statistics on job execution time.
 * - `out*` {Object, default: {inline:1}} sets the output target for the map reduce job.
 *
 * ####* out options:
 *
 * - `{inline:1}` the results are returned in an array
 * - `{replace: 'collectionName'}` add the results to collectionName: the results replace the collection
 * - `{reduce: 'collectionName'}` add the results to collectionName: if dups are detected, uses the reducer / finalize functions
 * - `{merge: 'collectionName'}` add the results to collectionName: if dups exist the new docs overwrite the old
 *
 * If `options.out` is set to `replace`, `merge`, or `reduce`, a Model instance is returned that can be used for further querying. Queries run against this model are all executed with the `lean` option; meaning only the js object is returned and no Mongoose magic is applied (getters, setters, etc).
 *
 * ####Example:
 *
 *     var o = {};
 *     o.map = function () { emit(this.name, 1) }
 *     o.reduce = function (k, vals) { return vals.length }
 *     o.out = { replace: 'createdCollectionNameForResults' }
 *     o.verbose = true;
 *     User.mapReduce(o, function (err, model, stats) {
 *       console.log('map reduce took %d ms', stats.processtime)
 *       model.find().where('value').gt(10).exec(function (err, docs) {
 *         console.log(docs);
 *       });
 *     })
 *
 * @param {Object} o an object specifying map-reduce options
 * @param {Function} callback
 * @see http://www.mongodb.org/display/DOCS/MapReduce
 * @api public
 */

Model.mapReduce = function mapReduce (o, callback) {
  if ('function' != typeof callback) throw new Error('missing callback');

  var self = this;

  if (!Model.mapReduce.schema) {
    var opts = { noId: true, noVirtualId: true, strict: false }
    Model.mapReduce.schema = new Schema({}, opts);
  }

  if (!o.out) o.out = { inline: 1 };

  o.map = String(o.map);
  o.reduce = String(o.reduce);

  if (o.query) {
    var q = new Query(o.query);
    q.cast(this);
    o.query = q._conditions;
    q = undefined;
  }

  this.collection.mapReduce(null, null, o, function (err, ret, stats) {
    if (err) return callback(err);

    if (ret.findOne && ret.mapReduce) {
      // returned a collection, convert to Model
      var model = Model.compile(
          '_mapreduce_' + ret.collectionName
        , Model.mapReduce.schema
        , ret.collectionName
        , self.db
        , self.base);

      model._mapreduce = true;

      return callback(err, model, stats);
    }

    callback(err, ret, stats);
  });
}

/**
 * Executes an aggregate command on this models collection.
 *
 * ####Example:
 *
 *     // find the max age of all users
 *     Users.aggregate(
 *         { $group: { _id: null, maxAge: { $max: '$age' }}}
 *       , { $project: { _id: 0, maxAge: 1 }}
 *       , function (err, res) {
 *       if (err) return handleError(err);
 *       console.log(res); // [ { maxAge: 98 } ]
 *     });
 *
 * ####NOTE:
 *
 * - At this time, arguments are not cast to the schema because $project operators allow redefining the "shape" of the documents at any stage of the pipeline.
 * - The documents returned are plain javascript objects, not mongoose documents cast to this models schema definition (since any shape of document can be returned).
 * - Requires MongoDB >= 2.1
 *
 * @param {Array} array an array of pipeline commands
 * @param {Object} [options]
 * @param {Function} callback
 * @see aggregation http://docs.mongodb.org/manual/applications/aggregation/
 * @see driver http://mongodb.github.com/node-mongodb-native/api-generated/collection.html#aggregate
 * @api public
 */

Model.aggregate = function aggregate () {
  return this.collection.aggregate.apply(this.collection, arguments);
}

/**
 * Populates document references.
 *
 * ####Available options:
 *
 * - path: space delimited path(s) to populate
 * - select: optional fields to select
 * - match: optional query conditions to match
 * - model: optional name of the model to use for population
 * - options: optional query options like sort, limit, etc
 *
 * ####Examples:
 *
 *     // populates a single object
 *     User.findById(id, function (err, user) {
 *       var opts = [
 *           { path: 'company', match: { x: 1 }, select: 'name' }
 *         , { path: 'notes', options: { limit: 10 }, model: 'override' }
 *       ]
 *
 *       User.populate(user, opts, function (err, user) {
 *         console.log(user);
 *       })
 *     })
 *
 *     // populates an array of objects
 *     User.find(match, function (err, users) {
 *       var opts = [{ path: 'company', match: { x: 1 }, select: 'name' }]
 *
 *       User.populate(users, opts, function (err, user) {
 *         console.log(user);
 *       })
 *     })
 *
 *     // imagine a Weapon model exists with two saved documents:
 *     //   { _id: 389, name: 'whip' }
 *     //   { _id: 8921, name: 'boomerang' }
 *
 *     var user = { name: 'Indiana Jones', weapon: 389 }
 *     Weapon.populate(user, { path: 'weapon', model: 'Weapon' }, function (err, user) {
 *       console.log(user.weapon.name) // whip
 *     })
 *
 *     // populate many plain objects
 *     var users = [{ name: 'Indiana Jones', weapon: 389 }]
 *     users.push({ name: 'Batman', weapon: 8921 })
 *     Weapon.populate(users, { path: 'weapon' }, function (err, users) {
 *       users.forEach(function (user) {
 *         console.log('%s uses a %s', users.name, user.weapon.name)
 *         // Indiana Jones uses a whip
 *         // Batman uses a boomerang
 *       })
 *     })
 *     // Note that we didn't need to specify the Weapon model because
 *     // we were already using it's populate() method.
 *
 * @param {Document|Array} docs Either a single document or array of documents to populate.
 * @param {Object} options A hash of key/val (path, options) used for population.
 * @param {Function} cb(err,doc) A callback, executed upon completion. Receives `err` and the `doc(s)`.
 * @api public
 */

Model.populate = function (docs, paths, cb) {
  assert.equal('function', typeof cb);

  // always callback on nextTick for consistent async behavior
  function callback () {
    var args = utils.args(arguments);
    process.nextTick(function () {
      cb.apply(null, args);
    });
  }

  // normalized paths
  var paths = utils.populate(paths);
  var pending = paths.length;

  if (0 === pending) {
    return callback(null, docs);
  }

  // each path has its own query options and must be executed separately
  var i = pending;
  var path;
  while (i--) {
    path = paths[i];
    populate(this, docs, path, next);
  }

  function next (err) {
    if (next.err) return;
    if (err) return callback(next.err = err);
    if (--pending) return;
    callback(null, docs);
  }
}

/*!
 * Prepare population options
 */

function populate (model, docs, options, cb) {
  var path = options.path
  var schema = model._getSchema(path);

  // handle document arrays
  if (schema && schema.caster) {
    schema = schema.caster;
  }

  // model name for the populate query
  var modelName = options.model && options.model.modelName
               || options.model                // query options
               || schema && schema.options.ref // declared in schema
               || model.modelName              // an ad-hoc structure

  var Model = model.db.model(modelName);

  // expose the model used
  options.model = Model;

  // normalize single / multiple docs passed
  if (!Array.isArray(docs)) {
    docs = [docs];
  }

  if (0 === docs.length || docs.every(utils.isNullOrUndefined)) {
    return cb();
  }

  populateDocs(docs, options, cb);
}

/*!
 * Populates `docs`
 */

function populateDocs (docs, options, cb) {
  var select = options.select;
  var match = options.match;
  var path = options.path;
  var Model = options.model;

  var rawIds = [];
  var i, doc, id;
  var len = docs.length;
  var found = 0;
  var isDocument;
  var subpath;
  var ret;

  for (i = 0; i < len; i++) {
    ret = undefined;
    doc = docs[i];
    id = String(utils.getValue("_id", doc));
    isDocument = !! doc.$__;

    if (isDocument && !doc.isModified(path)) {
      // it is possible a previously populated path is being
      // populated again. Because users can specify matcher
      // clauses in their populate arguments we use the cached
      // _ids from the original populate call to ensure all _ids
      // are looked up, but only if the path wasn't modified which
      // signifies the users intent of the state of the path.
      ret = doc.populated(path);
    }

    if (!ret || Array.isArray(ret) && 0 === ret.length) {
      ret = utils.getValue(path, doc);
    }

    if (ret) {
      ret = convertTo_id(ret);

      // previously we always assigned this even if the document had no _id
      options._docs[id] = Array.isArray(ret)
        ? ret.slice()
        : ret;
    }

    // always retain original values, even empty values. these are
    // used to map the query results back to the correct position.
    rawIds.push(ret);

    if (isDocument) {
      // cache original populated _ids and model used
      doc.populated(path, options._docs[id], options);
    }
  }

  var ids = utils.array.flatten(rawIds, function (item) {
    // no need to include undefined values in our query
    return undefined !== item;
  });

  if (0 === ids.length || ids.every(utils.isNullOrUndefined)) {
    return cb();
  }

  // preserve original match conditions by copying
  if (match) {
    match = utils.object.shallowCopy(match);
  } else {
    match = {};
  }

  match._id || (match._id = { $in: ids });

  var assignmentOpts = {};
  assignmentOpts.sort = options.options && options.options.sort || undefined;
  assignmentOpts.excludeId = /\s?-_id\s?/.test(select) || (select && 0 === select._id);

  if (assignmentOpts.excludeId) {
    // override the exclusion from the query so we can use the _id
    // for document matching during assignment. we'll delete the
    // _id back off before returning the result.
    if ('string' == typeof select) {
      select = select.replace(/\s?-_id\s?/g, ' ');
    } else {
      // preserve original select conditions by copying
      select = utils.object.shallowCopy(select);
      delete select._id;
    }
  }

  // if a limit option is passed, we should have the limit apply to *each*
  // document, not apply in the aggregate
  if (options.options && options.options.limit) {
    options.options.limit = options.options.limit * len;
  }

  Model.find(match, select, options.options, function (err, vals) {
    if (err) return cb(err);

    var lean = options.options && options.options.lean;
    var len = vals.length;
    var rawOrder = {};
    var rawDocs = {}
    var key;
    var val;

    // optimization:
    // record the document positions as returned by
    // the query result.
    for (var i = 0; i < len; i++) {
      val = vals[i];
      key = String(utils.getValue('_id', val));
      rawDocs[key] = val;
      rawOrder[key] = i;

      // flag each as result of population
      if (!lean) val.$__.wasPopulated = true;
    }

    assignVals({
      rawIds: rawIds,
      rawDocs: rawDocs,
      rawOrder: rawOrder,
      docs: docs,
      path: path,
      options: assignmentOpts
    });

    cb();
  });
}

/*!
 * Retrieve the _id of `val` if a Document or Array of Documents.
 *
 * @param {Array|Document|Any} val
 * @return {Array|Document|Any}
 */

function convertTo_id (val) {
  if (val instanceof Model) return val._id;

  if (Array.isArray(val)) {
    for (var i = 0; i < val.length; ++i) {
      if (val[i] instanceof Model) {
        val[i] = val[i]._id;
      }
    }
    return val;
  }

  return val;
}

/*!
 * Assigns documents returned from a population query back
 * to the original document path.
 */

function assignVals (o) {
  // replace the original ids in our intermediate _ids structure
  // with the documents found by query

  assignRawDocsToIdStructure(o.rawIds, o.rawDocs, o.rawOrder, o.options);

  // now update the original documents being populated using the
  // result structure that contains real documents.

  var docs = o.docs;
  var path = o.path;
  var rawIds = o.rawIds;
  var options = o.options;

  for (var i = 0; i < docs.length; ++i) {
    utils.setValue(path, rawIds[i], docs[i], function (val) {
      return valueFilter(val, options);
    });
  }
}

/*!
 * 1) Apply backwards compatible find/findOne behavior to sub documents
 *
 *    find logic:
 *      a) filter out non-documents
 *      b) remove _id from sub docs when user specified
 *
 *    findOne
 *      a) if no doc found, set to null
 *      b) remove _id from sub docs when user specified
 *
 * 2) Remove _ids when specified by users query.
 *
 * background:
 * _ids are left in the query even when user excludes them so
 * that population mapping can occur.
 */

function valueFilter (val, assignmentOpts) {
  if (Array.isArray(val)) {
    // find logic
    var ret = [];
    for (var i = 0; i < val.length; ++i) {
      var subdoc = val[i];
      if (!isDoc(subdoc)) continue;
      maybeRemoveId(subdoc, assignmentOpts);
      ret.push(subdoc);
    }
    return ret;
  }

  // findOne
  if (isDoc(val)) {
    maybeRemoveId(val, assignmentOpts);
    return val;
  }

  return null;
}

/*!
 * Remove _id from `subdoc` if user specified "lean" query option
 */

function maybeRemoveId (subdoc, assignmentOpts) {
  if (assignmentOpts.excludeId) {
    if ('function' == typeof subdoc.setValue) {
      subdoc.setValue('_id', undefined);
    } else {
      delete subdoc._id;
    }
  }
}

/*!
 * Determine if `doc` is a document returned
 * by a populate query.
 */

function isDoc (doc) {
  if (null == doc)
    return false;

  var type = typeof doc;
  if ('string' == type)
    return false;

  if ('number' == type)
    return false;

  if (Buffer.isBuffer(doc))
    return false;

  if ('ObjectID' == doc.constructor.name)
    return false;

  // only docs
  return true;
}

/*!
 * Assign `vals` returned by mongo query to the `rawIds`
 * structure returned from utils.getVals() honoring
 * query sort order if specified by user.
 *
 * This can be optimized.
 *
 * Rules:
 *
 *   if the value of the path is not an array, use findOne rules, else find.
 *   for findOne the results are assigned directly to doc path (including null results).
 *   for find, if user specified sort order, results are assigned directly
 *   else documents are put back in original order of array if found in results
 *
 * @param {Array} rawIds
 * @param {Array} vals
 * @param {Boolean} sort
 * @api private
 */

function assignRawDocsToIdStructure (rawIds, resultDocs, resultOrder, options, recursed) {
  // honor user specified sort order
  var newOrder = [];
  var sorting = options.sort && rawIds.length > 1;
  var found;
  var doc;
  var sid;
  var id;

  for (var i = 0; i < rawIds.length; ++i) {
    id = rawIds[i];

    if (Array.isArray(id)) {
      // handle [ [id0, id2], [id3] ]
      assignRawDocsToIdStructure(id, resultDocs, resultOrder, options, true);
      newOrder.push(id);
      continue;
    }

    if (null === id && !sorting) {
      // keep nulls for findOne unless sorting, which always
      // removes them (backward compat)
      newOrder.push(id);
      continue;
    }

    sid = String(id);
    found = false;

    if (recursed) {
      // apply find behavior

      // assign matching documents in original order unless sorting
      doc = resultDocs[sid];
      if (doc) {
        if (sorting) {
          newOrder[resultOrder[sid]] = doc;
        } else {
          newOrder.push(doc);
        }
      } else {
        newOrder.push(id);
      }
    } else {
      // apply findOne behavior - if document in results, assign, else assign null
      newOrder[i] = doc = resultDocs[sid] || null;
    }
  }

  rawIds.length = 0;
  if (newOrder.length) {
    // reassign the documents based on corrected order

    // forEach skips over sparse entries in arrays so we
    // can safely use this to our advantage dealing with sorted
    // result sets too.
    newOrder.forEach(function (doc, i) {
      rawIds[i] = doc;
    });
  }
}

/**
 * Finds the schema for `path`. This is different than
 * calling `schema.path` as it also resolves paths with
 * positional selectors (something.$.another.$.path).
 *
 * @param {String} path
 * @return {Schema}
 * @api private
 */

Model._getSchema = function _getSchema (path) {
  var schema = this.schema
    , pathschema = schema.path(path);

  if (pathschema)
    return pathschema;

  // look for arrays
  return (function search (parts, schema) {
    var p = parts.length + 1
      , foundschema
      , trypath

    while (p--) {
      trypath = parts.slice(0, p).join('.');
      foundschema = schema.path(trypath);
      if (foundschema) {
        if (foundschema.caster) {

          // array of Mixed?
          if (foundschema.caster instanceof Types.Mixed) {
            return foundschema.caster;
          }

          // Now that we found the array, we need to check if there
          // are remaining document paths to look up for casting.
          // Also we need to handle array.$.path since schema.path
          // doesn't work for that.
          // If there is no foundschema.schema we are dealing with
          // a path like array.$
          if (p !== parts.length && foundschema.schema) {
            if ('$' === parts[p]) {
              // comments.$.comments.$.title
              return search(parts.slice(p+1), foundschema.schema);
            } else {
              // this is the last path of the selector
              return search(parts.slice(p), foundschema.schema);
            }
          }
        }
        return foundschema;
      }
    }
  })(path.split('.'), schema)
}

/*!
 * Compiler utility.
 *
 * @param {String} name model name
 * @param {Schema} schema
 * @param {String} collectionName
 * @param {Connection} connection
 * @param {Mongoose} base mongoose instance
 */

Model.compile = function compile (name, schema, collectionName, connection, base) {
  var versioningEnabled = false !== schema.options.versionKey;

  if (versioningEnabled && !schema.paths[schema.options.versionKey]) {
    // add versioning to top level documents only
    var o = {};
    o[schema.options.versionKey] = Number;
    schema.add(o);
  }

  // generate new class
  function model (doc, fields, skipId) {
    if (!(this instanceof model))
      return new model(doc, fields, skipId);
    Model.call(this, doc, fields, skipId);
  };

  model.base = base;
  model.modelName = name;
  model.__proto__ = Model;
  model.prototype.__proto__ = Model.prototype;
  model.model = Model.prototype.model;
  model.db = model.prototype.db = connection;

  model.prototype.$__setSchema(schema);

  var collectionOptions = {
      bufferCommands: schema.options.bufferCommands
    , capped: schema.options.capped
  };

  model.prototype.collection = connection.collection(
      collectionName
    , collectionOptions
  );

  // apply methods
  for (var i in schema.methods)
    model.prototype[i] = schema.methods[i];

  // apply statics
  for (var i in schema.statics)
    model[i] = schema.statics[i];

  // apply named scopes
  if (schema.namedScopes) schema.namedScopes.compile(model);

  model.schema = model.prototype.schema;
  model.options = model.prototype.options;
  model.collection = model.prototype.collection;

  return model;
};

/*!
 * Subclass this model with `conn`, `schema`, and `collection` settings.
 *
 * @param {Connection} conn
 * @param {Schema} [schema]
 * @param {String} [collection]
 * @return {Model}
 */

Model.__subclass = function subclass (conn, schema, collection) {
  // subclass model using this connection and collection name
  var model = this;

  var Model = function Model (doc, fields, skipId) {
    if (!(this instanceof Model)) {
      return new Model(doc, fields, skipId);
    }
    model.call(this, doc, fields, skipId);
  }

  Model.__proto__ = model;
  Model.prototype.__proto__ = model.prototype;
  Model.db = Model.prototype.db = conn;

  var s = schema && 'string' != typeof schema
    ? schema
    : model.prototype.schema;

  if (!collection) {
    collection = model.prototype.schema.get('collection')
              || utils.toCollectionName(model.modelName);
  }

  var collectionOptions = {
      bufferCommands: s ? s.options.bufferCommands : true
    , capped: s && s.options.capped
  };

  Model.prototype.collection = conn.collection(collection, collectionOptions);
  Model.collection = Model.prototype.collection;
  Model.init();
  return Model;
}

/*!
 * Module exports.
 */

module.exports = exports = Model;