'use strict';

/*!
 * Module dependencies.
 */

const ChangeStream = require('./cursor/ChangeStream');
const EventEmitter = require('events').EventEmitter;
const Schema = require('./schema');
const Collection = require('./driver').get().Collection;
const STATES = require('./connectionstate');
const MongooseError = require('./error/index');
const PromiseProvider = require('./promise_provider');
const ServerSelectionError = require('./error/serverSelection');
const applyPlugins = require('./helpers/schema/applyPlugins');
const promiseOrCallback = require('./helpers/promiseOrCallback');
const get = require('./helpers/get');
const immediate = require('./helpers/immediate');
const mongodb = require('mongodb');
const pkg = require('../package.json');
const utils = require('./utils');

const parseConnectionString = require('mongodb/lib/core').parseConnectionString;

let id = 0;

/*!
 * A list of authentication mechanisms that don't require a password for authentication.
 * This is used by the authMechanismDoesNotRequirePassword method.
 *
 * @api private
 */
const noPasswordAuthMechanisms = [
  'MONGODB-X509'
];

/**
 * Connection constructor
 *
 * For practical reasons, a Connection equals a Db.
 *
 * @param {Mongoose} base a mongoose instance
 * @inherits NodeJS EventEmitter http://nodejs.org/api/events.html#events_class_events_eventemitter
 * @event `connecting`: Emitted when `connection.openUri()` is executed on this connection.
 * @event `connected`: Emitted when this connection successfully connects to the db. May be emitted _multiple_ times in `reconnected` scenarios.
 * @event `open`: Emitted after we `connected` and `onOpen` is executed on all of this connections models.
 * @event `disconnecting`: Emitted when `connection.close()` was executed.
 * @event `disconnected`: Emitted after getting disconnected from the db.
 * @event `close`: Emitted after we `disconnected` and `onClose` executed on all of this connections models.
 * @event `reconnected`: Emitted after we `connected` and subsequently `disconnected`, followed by successfully another successful connection.
 * @event `error`: Emitted when an error occurs on this connection.
 * @event `fullsetup`: Emitted after the driver has connected to primary and all secondaries if specified in the connection string.
 * @api public
 */

function Connection(base) {
  this.base = base;
  this.collections = {};
  this.models = {};
  this.config = { autoIndex: true };
  this.replica = false;
  this.options = null;
  this.otherDbs = []; // FIXME: To be replaced with relatedDbs
  this.relatedDbs = {}; // Hashmap of other dbs that share underlying connection
  this.states = STATES;
  this._readyState = STATES.disconnected;
  this._closeCalled = false;
  this._hasOpened = false;
  this.plugins = [];
  this.id = id++;
}

/*!
 * Inherit from EventEmitter
 */

Connection.prototype.__proto__ = EventEmitter.prototype;

/**
 * Connection ready state
 *
 * - 0 = disconnected
 * - 1 = connected
 * - 2 = connecting
 * - 3 = disconnecting
 *
 * Each state change emits its associated event name.
 *
 * ####Example
 *
 *     conn.on('connected', callback);
 *     conn.on('disconnected', callback);
 *
 * @property readyState
 * @memberOf Connection
 * @instance
 * @api public
 */

Object.defineProperty(Connection.prototype, 'readyState', {
  get: function() {
    return this._readyState;
  },
  set: function(val) {
    if (!(val in STATES)) {
      throw new Error('Invalid connection state: ' + val);
    }

    if (this._readyState !== val) {
      this._readyState = val;
      // [legacy] loop over the otherDbs on this connection and change their state
      for (const db of this.otherDbs) {
        db.readyState = val;
      }

      // loop over relatedDbs on this connection and change their state
      for (const k in this.relatedDbs) {
        this.relatedDbs[k].readyState = val;
      }

      if (STATES.connected === val) {
        this._hasOpened = true;
      }

      this.emit(STATES[val]);
    }
  }
});

/**
 * Gets the value of the option `key`. Equivalent to `conn.options[key]`
 *
 * ####Example:
 *
 *     conn.get('test'); // returns the 'test' value
 *
 * @param {String} key
 * @method get
 * @api public
 */

Connection.prototype.get = function(key) {
  return get(this.options, key);
};

/**
 * Sets the value of the option `key`. Equivalent to `conn.options[key] = val`
 *
 * Supported options include:
 *
 * - `maxTimeMS`: Set [`maxTimeMS`](/docs/api.html#query_Query-maxTimeMS) for all queries on this connection.
 * - `useFindAndModify`: Set to `false` to work around the [`findAndModify()` deprecation warning](/docs/deprecations.html#findandmodify)
 *
 * ####Example:
 *
 *     conn.set('test', 'foo');
 *     conn.get('test'); // 'foo'
 *     conn.options.test; // 'foo'
 *
 * @param {String} key
 * @param {Any} val
 * @method set
 * @api public
 */

Connection.prototype.set = function(key, val) {
  this.options = this.options || {};
  this.options[key] = val;
  return val;
};

/**
 * A hash of the collections associated with this connection
 *
 * @property collections
 * @memberOf Connection
 * @instance
 * @api public
 */

Connection.prototype.collections;

/**
 * The name of the database this connection points to.
 *
 * ####Example
 *
 *     mongoose.createConnection('mongodb://localhost:27017/mydb').name; // "mydb"
 *
 * @property name
 * @memberOf Connection
 * @instance
 * @api public
 */

Connection.prototype.name;

/**
 * A [POJO](https://masteringjs.io/tutorials/fundamentals/pojo) containing
 * a map from model names to models. Contains all models that have been
 * added to this connection using [`Connection#model()`](/docs/api/connection.html#connection_Connection-model).
 *
 * ####Example
 *
 *     const conn = mongoose.createConnection();
 *     const Test = conn.model('Test', mongoose.Schema({ name: String }));
 *
 *     Object.keys(conn.models).length; // 1
 *     conn.models.Test === Test; // true
 *
 * @property models
 * @memberOf Connection
 * @instance
 * @api public
 */

Connection.prototype.models;

/**
 * A number identifier for this connection. Used for debugging when
 * you have [multiple connections](/docs/connections.html#multiple_connections).
 *
 * ####Example
 *
 *     // The default connection has `id = 0`
 *     mongoose.connection.id; // 0
 *
 *     // If you create a new connection, Mongoose increments id
 *     const conn = mongoose.createConnection();
 *     conn.id; // 1
 *
 * @property id
 * @memberOf Connection
 * @instance
 * @api public
 */

Connection.prototype.id;

/**
 * The plugins that will be applied to all models created on this connection.
 *
 * ####Example:
 *
 *     const db = mongoose.createConnection('mongodb://localhost:27017/mydb');
 *     db.plugin(() => console.log('Applied'));
 *     db.plugins.length; // 1
 *
 *     db.model('Test', new Schema({})); // Prints "Applied"
 *
 * @property plugins
 * @memberOf Connection
 * @instance
 * @api public
 */

Object.defineProperty(Connection.prototype, 'plugins', {
  configurable: false,
  enumerable: true,
  writable: true
});

/**
 * The host name portion of the URI. If multiple hosts, such as a replica set,
 * this will contain the first host name in the URI
 *
 * ####Example
 *
 *     mongoose.createConnection('mongodb://localhost:27017/mydb').host; // "localhost"
 *
 * @property host
 * @memberOf Connection
 * @instance
 * @api public
 */

Object.defineProperty(Connection.prototype, 'host', {
  configurable: true,
  enumerable: true,
  writable: true
});

/**
 * The port portion of the URI. If multiple hosts, such as a replica set,
 * this will contain the port from the first host name in the URI.
 *
 * ####Example
 *
 *     mongoose.createConnection('mongodb://localhost:27017/mydb').port; // 27017
 *
 * @property port
 * @memberOf Connection
 * @instance
 * @api public
 */

Object.defineProperty(Connection.prototype, 'port', {
  configurable: true,
  enumerable: true,
  writable: true
});

/**
 * The username specified in the URI
 *
 * ####Example
 *
 *     mongoose.createConnection('mongodb://val:psw@localhost:27017/mydb').user; // "val"
 *
 * @property user
 * @memberOf Connection
 * @instance
 * @api public
 */

Object.defineProperty(Connection.prototype, 'user', {
  configurable: true,
  enumerable: true,
  writable: true
});

/**
 * The password specified in the URI
 *
 * ####Example
 *
 *     mongoose.createConnection('mongodb://val:psw@localhost:27017/mydb').pass; // "psw"
 *
 * @property pass
 * @memberOf Connection
 * @instance
 * @api public
 */

Object.defineProperty(Connection.prototype, 'pass', {
  configurable: true,
  enumerable: true,
  writable: true
});

/**
 * The mongodb.Db instance, set when the connection is opened
 *
 * @property db
 * @memberOf Connection
 * @instance
 * @api public
 */

Connection.prototype.db;

/**
 * A hash of the global options that are associated with this connection
 *
 * @property config
 * @memberOf Connection
 * @instance
 * @api public
 */

Connection.prototype.config;

/**
 * Helper for `createCollection()`. Will explicitly create the given collection
 * with specified options. Used to create [capped collections](https://docs.mongodb.com/manual/core/capped-collections/)
 * and [views](https://docs.mongodb.com/manual/core/views/) from mongoose.
 *
 * Options are passed down without modification to the [MongoDB driver's `createCollection()` function](http://mongodb.github.io/node-mongodb-native/2.2/api/Db.html#createCollection)
 *
 * @method createCollection
 * @param {string} collection The collection to create
 * @param {Object} [options] see [MongoDB driver docs](http://mongodb.github.io/node-mongodb-native/2.2/api/Db.html#createCollection)
 * @param {Function} [callback]
 * @return {Promise}
 * @api public
 */

Connection.prototype.createCollection = _wrapConnHelper(function createCollection(collection, options, cb) {
  if (typeof options === 'function') {
    cb = options;
    options = {};
  }
  this.db.createCollection(collection, options, cb);
});

/**
 * _Requires MongoDB >= 3.6.0._ Starts a [MongoDB session](https://docs.mongodb.com/manual/release-notes/3.6/#client-sessions)
 * for benefits like causal consistency, [retryable writes](https://docs.mongodb.com/manual/core/retryable-writes/),
 * and [transactions](http://thecodebarbarian.com/a-node-js-perspective-on-mongodb-4-transactions.html).
 *
 * ####Example:
 *
 *     const session = await conn.startSession();
 *     let doc = await Person.findOne({ name: 'Ned Stark' }, null, { session });
 *     await doc.remove();
 *     // `doc` will always be null, even if reading from a replica set
 *     // secondary. Without causal consistency, it is possible to
 *     // get a doc back from the below query if the query reads from a
 *     // secondary that is experiencing replication lag.
 *     doc = await Person.findOne({ name: 'Ned Stark' }, null, { session, readPreference: 'secondary' });
 *
 *
 * @method startSession
 * @param {Object} [options] see the [mongodb driver options](http://mongodb.github.io/node-mongodb-native/3.0/api/MongoClient.html#startSession)
 * @param {Boolean} [options.causalConsistency=true] set to false to disable causal consistency
 * @param {Function} [callback]
 * @return {Promise<ClientSession>} promise that resolves to a MongoDB driver `ClientSession`
 * @api public
 */

Connection.prototype.startSession = _wrapConnHelper(function startSession(options, cb) {
  if (typeof options === 'function') {
    cb = options;
    options = null;
  }
  const session = this.client.startSession(options);
  cb(null, session);
});

/**
 * Helper for `dropCollection()`. Will delete the given collection, including
 * all documents and indexes.
 *
 * @method dropCollection
 * @param {string} collection The collection to delete
 * @param {Function} [callback]
 * @return {Promise}
 * @api public
 */

Connection.prototype.dropCollection = _wrapConnHelper(function dropCollection(collection, cb) {
  this.db.dropCollection(collection, cb);
});

/**
 * Helper for `dropDatabase()`. Deletes the given database, including all
 * collections, documents, and indexes.
 *
 * ####Example:
 *
 *     const conn = mongoose.createConnection('mongodb://localhost:27017/mydb');
 *     // Deletes the entire 'mydb' database
 *     await conn.dropDatabase();
 *
 * @method dropDatabase
 * @param {Function} [callback]
 * @return {Promise}
 * @api public
 */

Connection.prototype.dropDatabase = _wrapConnHelper(function dropDatabase(cb) {
  // If `dropDatabase()` is called, this model's collection will not be
  // init-ed. It is sufficiently common to call `dropDatabase()` after
  // `mongoose.connect()` but before creating models that we want to
  // support this. See gh-6967
  for (const name of Object.keys(this.models)) {
    delete this.models[name].$init;
  }
  this.db.dropDatabase(cb);
});

/*!
 * ignore
 */

function _wrapConnHelper(fn) {
  return function() {
    const cb = arguments.length > 0 ? arguments[arguments.length - 1] : null;
    const argsWithoutCb = typeof cb === 'function' ?
      Array.prototype.slice.call(arguments, 0, arguments.length - 1) :
      Array.prototype.slice.call(arguments);
    const disconnectedError = new MongooseError('Connection ' + this.id +
      ' was disconnected when calling `' + fn.name + '`');
    return promiseOrCallback(cb, cb => {
      // Make it ok to call collection helpers before `mongoose.connect()`
      // as long as `mongoose.connect()` is called on the same tick.
      // Re: gh-8534
      immediate(() => {
        if (this.readyState === STATES.connecting) {
          this.once('open', function() {
            fn.apply(this, argsWithoutCb.concat([cb]));
          });
        } else if (this.readyState === STATES.disconnected && this.db == null) {
          cb(disconnectedError);
        } else {
          fn.apply(this, argsWithoutCb.concat([cb]));
        }
      });
    });
  };
}

/**
 * error
 *
 * Graceful error handling, passes error to callback
 * if available, else emits error on the connection.
 *
 * @param {Error} err
 * @param {Function} callback optional
 * @api private
 */

Connection.prototype.error = function(err, callback) {
  if (callback) {
    callback(err);
    return null;
  }
  if (this.listeners('error').length > 0) {
    this.emit('error', err);
  }
  return Promise.reject(err);
};

/**
 * Called when the connection is opened
 *
 * @api private
 */

Connection.prototype.onOpen = function() {
  this.readyState = STATES.connected;

  // avoid having the collection subscribe to our event emitter
  // to prevent 0.3 warning
  for (const i in this.collections) {
    if (utils.object.hasOwnProperty(this.collections, i)) {
      this.collections[i].onOpen();
    }
  }

  this.emit('open');
};

/**
 * Opens the connection with a URI using `MongoClient.connect()`.
 *
 * @param {String} uri The URI to connect with.
 * @param {Object} [options] Passed on to http://mongodb.github.io/node-mongodb-native/2.2/api/MongoClient.html#connect
 * @param {Boolean} [options.bufferCommands=true] Mongoose specific option. Set to false to [disable buffering](http://mongoosejs.com/docs/faq.html#callback_never_executes) on all models associated with this connection.
 * @param {String} [options.dbName] The name of the database we want to use. If not provided, use database name from connection string.
 * @param {String} [options.user] username for authentication, equivalent to `options.auth.user`. Maintained for backwards compatibility.
 * @param {String} [options.pass] password for authentication, equivalent to `options.auth.password`. Maintained for backwards compatibility.
 * @param {Number} [options.poolSize=5] The maximum number of sockets the MongoDB driver will keep open for this connection. By default, `poolSize` is 5. Keep in mind that, as of MongoDB 3.4, MongoDB only allows one operation per socket at a time, so you may want to increase this if you find you have a few slow queries that are blocking faster queries from proceeding. See [Slow Trains in MongoDB and Node.js](http://thecodebarbarian.com/slow-trains-in-mongodb-and-nodejs).
 * @param {Boolean} [options.useUnifiedTopology=false] False by default. Set to `true` to opt in to the MongoDB driver's replica set and sharded cluster monitoring engine.
 * @param {Number} [options.serverSelectionTimeoutMS] If `useUnifiedTopology = true`, the MongoDB driver will try to find a server to send any given operation to, and keep retrying for `serverSelectionTimeoutMS` milliseconds before erroring out. If not set, the MongoDB driver defaults to using `30000` (30 seconds).
 * @param {Number} [options.heartbeatFrequencyMS] If `useUnifiedTopology = true`, the MongoDB driver sends a heartbeat every `heartbeatFrequencyMS` to check on the status of the connection. A heartbeat is subject to `serverSelectionTimeoutMS`, so the MongoDB driver will retry failed heartbeats for up to 30 seconds by default. Mongoose only emits a `'disconnected'` event after a heartbeat has failed, so you may want to decrease this setting to reduce the time between when your server goes down and when Mongoose emits `'disconnected'`. We recommend you do **not** set this setting below 1000, too many heartbeats can lead to performance degradation.
 * @param {Boolean} [options.autoIndex=true] Mongoose-specific option. Set to false to disable automatic index creation for all models associated with this connection.
 * @param {Boolean} [options.useNewUrlParser=false] False by default. Set to `true` to opt in to the MongoDB driver's new URL parser logic.
 * @param {Boolean} [options.useCreateIndex=true] Mongoose-specific option. If `true`, this connection will use [`createIndex()` instead of `ensureIndex()`](/docs/deprecations.html#ensureindex) for automatic index builds via [`Model.init()`](/docs/api.html#model_Model.init).
 * @param {Boolean} [options.useFindAndModify=true] True by default. Set to `false` to make `findOneAndUpdate()` and `findOneAndRemove()` use native `findOneAndUpdate()` rather than `findAndModify()`.
 * @param {Number} [options.reconnectTries=30] If you're connected to a single server or mongos proxy (as opposed to a replica set), the MongoDB driver will try to reconnect every `reconnectInterval` milliseconds for `reconnectTries` times, and give up afterward. When the driver gives up, the mongoose connection emits a `reconnectFailed` event. This option does nothing for replica set connections.
 * @param {Number} [options.reconnectInterval=1000] See `reconnectTries` option above.
 * @param {Class} [options.promiseLibrary] Sets the [underlying driver's promise library](http://mongodb.github.io/node-mongodb-native/3.1/api/MongoClient.html).
 * @param {Number} [options.bufferMaxEntries] This option does nothing if `useUnifiedTopology` is set. The MongoDB driver also has its own buffering mechanism that kicks in when the driver is disconnected. Set this option to 0 and set `bufferCommands` to `false` on your schemas if you want your database operations to fail immediately when the driver is not connected, as opposed to waiting for reconnection.
 * @param {Number} [options.connectTimeoutMS=30000] How long the MongoDB driver will wait before killing a socket due to inactivity _during initial connection_. Defaults to 30000. This option is passed transparently to [Node.js' `socket#setTimeout()` function](https://nodejs.org/api/net.html#net_socket_settimeout_timeout_callback).
 * @param {Number} [options.socketTimeoutMS=30000] How long the MongoDB driver will wait before killing a socket due to inactivity _after initial connection_. A socket may be inactive because of either no activity or a long-running operation. This is set to `30000` by default, you should set this to 2-3x your longest running operation if you expect some of your database operations to run longer than 20 seconds. This option is passed to [Node.js `socket#setTimeout()` function](https://nodejs.org/api/net.html#net_socket_settimeout_timeout_callback) after the MongoDB driver successfully completes.
 * @param {Number} [options.family=0] Passed transparently to [Node.js' `dns.lookup()`](https://nodejs.org/api/dns.html#dns_dns_lookup_hostname_options_callback) function. May be either `0, `4`, or `6`. `4` means use IPv4 only, `6` means use IPv6 only, `0` means try both.
 * @param {Function} [callback]
 * @returns {Connection} this
 * @api public
 */

Connection.prototype.openUri = function(uri, options, callback) {
  this.readyState = STATES.connecting;
  this._closeCalled = false;

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

  if (['string', 'number'].indexOf(typeof options) !== -1) {
    throw new MongooseError('Mongoose 5.x no longer supports ' +
      '`mongoose.connect(host, dbname, port)` or ' +
      '`mongoose.createConnection(host, dbname, port)`. See ' +
      'http://mongoosejs.com/docs/connections.html for supported connection syntax');
  }

  if (typeof uri !== 'string') {
    throw new MongooseError('The `uri` parameter to `openUri()` must be a ' +
      `string, got "${typeof uri}". Make sure the first parameter to ` +
      '`mongoose.connect()` or `mongoose.createConnection()` is a string.');
  }

  if (callback != null && typeof callback !== 'function') {
    throw new MongooseError('3rd parameter to `mongoose.connect()` or ' +
      '`mongoose.createConnection()` must be a function, got "' +
      typeof callback + '"');
  }

  const Promise = PromiseProvider.get();
  const _this = this;

  if (options) {
    options = utils.clone(options);
    const autoIndex = options.config && options.config.autoIndex != null ?
      options.config.autoIndex :
      options.autoIndex;
    if (autoIndex != null) {
      this.config.autoIndex = autoIndex !== false;
      delete options.config;
      delete options.autoIndex;
    }

    if ('autoCreate' in options) {
      this.config.autoCreate = !!options.autoCreate;
      delete options.autoCreate;
    }
    if ('useCreateIndex' in options) {
      this.config.useCreateIndex = !!options.useCreateIndex;
      delete options.useCreateIndex;
    }

    if ('useFindAndModify' in options) {
      this.config.useFindAndModify = !!options.useFindAndModify;
      delete options.useFindAndModify;
    }

    // Backwards compat
    if (options.user || options.pass) {
      options.auth = options.auth || {};
      options.auth.user = options.user;
      options.auth.password = options.pass;

      this.user = options.user;
      this.pass = options.pass;
    }
    delete options.user;
    delete options.pass;

    if (options.bufferCommands != null) {
      if (options.bufferMaxEntries == null) {
        options.bufferMaxEntries = 0;
      }
      this.config.bufferCommands = options.bufferCommands;
      delete options.bufferCommands;
    }

    if (options.useMongoClient != null) {
      handleUseMongoClient(options);
    }
  } else {
    options = {};
  }

  this._connectionOptions = options;
  const dbName = options.dbName;
  if (dbName != null) {
    this.$dbName = dbName;
  }
  delete options.dbName;

  if (!('promiseLibrary' in options)) {
    options.promiseLibrary = PromiseProvider.get();
  }
  if (!('useNewUrlParser' in options)) {
    if ('useNewUrlParser' in this.base.options) {
      options.useNewUrlParser = this.base.options.useNewUrlParser;
    } else {
      options.useNewUrlParser = false;
    }
  }
  if (!utils.hasUserDefinedProperty(options, 'useUnifiedTopology')) {
    if (utils.hasUserDefinedProperty(this.base.options, 'useUnifiedTopology')) {
      options.useUnifiedTopology = this.base.options.useUnifiedTopology;
    } else {
      options.useUnifiedTopology = false;
    }
  }
  if (!utils.hasUserDefinedProperty(options, 'driverInfo')) {
    options.driverInfo = {
      name: 'Mongoose',
      version: pkg.version
    };
  }

  const parsePromise = new Promise((resolve, reject) => {
    parseConnectionString(uri, options, (err, parsed) => {
      if (err) {
        return reject(err);
      }
      if (dbName) {
        this.name = dbName;
      } else if (parsed.defaultDatabase) {
        this.name = parsed.defaultDatabase;
      } else {
        this.name = get(parsed, 'auth.db', null);
      }
      this.host = get(parsed, 'hosts.0.host', 'localhost');
      this.port = get(parsed, 'hosts.0.port', 27017);
      this.user = this.user || get(parsed, 'auth.username');
      this.pass = this.pass || get(parsed, 'auth.password');
      resolve();
    });
  });

  const _handleReconnect = () => {
    // If we aren't disconnected, we assume this reconnect is due to a
    // socket timeout. If there's no activity on a socket for
    // `socketTimeoutMS`, the driver will attempt to reconnect and emit
    // this event.
    if (_this.readyState !== STATES.connected) {
      _this.readyState = STATES.connected;
      _this.emit('reconnect');
      _this.emit('reconnected');
    }
  };

  const promise = new Promise((resolve, reject) => {
    const client = new mongodb.MongoClient(uri, options);
    _this.client = client;
    client.connect(function(error) {
      if (error) {
        _this.readyState = STATES.disconnected;
        return reject(error);
      }

      const db = dbName != null ? client.db(dbName) : client.db();
      _this.db = db;

      // `useUnifiedTopology` events
      const type = get(db, 's.topology.s.description.type', '');
      if (options.useUnifiedTopology) {
        if (type === 'Single') {
          const server = Array.from(db.s.topology.s.servers.values())[0];

          server.s.topology.on('serverHeartbeatSucceeded', () => {
            _handleReconnect();
          });
          server.s.pool.on('reconnect', () => {
            _handleReconnect();
          });
          client.on('serverDescriptionChanged', ev => {
            const newDescription = ev.newDescription;
            if (newDescription.type === 'Standalone') {
              _handleReconnect();
            } else {
              _this.readyState = STATES.disconnected;
            }
          });
        } else if (type.startsWith('ReplicaSet')) {
          client.on('topologyDescriptionChanged', ev => {
            // Emit disconnected if we've lost connectivity to _all_ servers
            // in the replica set.
            const description = ev.newDescription;
            const servers = Array.from(ev.newDescription.servers.values());
            const allServersDisconnected = description.type === 'ReplicaSetNoPrimary' &&
              servers.reduce((cur, d) => cur || d.type === 'Unknown', false);
            if (_this.readyState === STATES.connected && allServersDisconnected) {
              // Implicitly emits 'disconnected'
              _this.readyState = STATES.disconnected;
            } else if (_this.readyState === STATES.disconnected && !allServersDisconnected) {
              _handleReconnect();
            }
          });

          db.on('close', function() {
            const type = get(db, 's.topology.s.description.type', '');
            if (type !== 'ReplicaSetWithPrimary') {
              // Implicitly emits 'disconnected'
              _this.readyState = STATES.disconnected;
            }
          });
        }
      }

      // Backwards compat for mongoose 4.x
      db.on('reconnect', function() {
        _handleReconnect();
      });
      db.s.topology.on('reconnectFailed', function() {
        _this.emit('reconnectFailed');
      });

      if (!options.useUnifiedTopology) {
        db.s.topology.on('left', function(data) {
          _this.emit('left', data);
        });
      }
      db.s.topology.on('joined', function(data) {
        _this.emit('joined', data);
      });
      db.s.topology.on('fullsetup', function(data) {
        _this.emit('fullsetup', data);
      });
      if (get(db, 's.topology.s.coreTopology.s.pool') != null) {
        db.s.topology.s.coreTopology.s.pool.on('attemptReconnect', function() {
          _this.emit('attemptReconnect');
        });
      }
      if (!options.useUnifiedTopology || !type.startsWith('ReplicaSet')) {
        db.on('close', function() {
          // Implicitly emits 'disconnected'
          _this.readyState = STATES.disconnected;
        });
      }

      if (!options.useUnifiedTopology) {
        client.on('left', function() {
          if (_this.readyState === STATES.connected &&
              get(db, 's.topology.s.coreTopology.s.replicaSetState.topologyType') === 'ReplicaSetNoPrimary') {
            _this.readyState = STATES.disconnected;
          }
        });
      }

      db.on('timeout', function() {
        _this.emit('timeout');
      });

      delete _this.then;
      delete _this.catch;
      _this.readyState = STATES.connected;

      for (const i in _this.collections) {
        if (utils.object.hasOwnProperty(_this.collections, i)) {
          _this.collections[i].onOpen();
        }
      }

      resolve(_this);
      _this.emit('open');
    });
  });

  const serverSelectionError = new ServerSelectionError();
  this.$initialConnection = Promise.all([promise, parsePromise]).
    then(res => res[0]).
    catch(err => {
      if (err != null && err.name === 'MongoServerSelectionError') {
        err = serverSelectionError.assimilateError(err);
      }

      if (this.listeners('error').length > 0) {
        process.nextTick(() => this.emit('error', err));
      }
      throw err;
    });
  this.then = function(resolve, reject) {
    return this.$initialConnection.then(resolve, reject);
  };
  this.catch = function(reject) {
    return this.$initialConnection.catch(reject);
  };

  if (callback != null) {
    this.$initialConnection = this.$initialConnection.then(
      () => callback(null, this),
      err => callback(err)
    );
  }

  return this;
};

/*!
 * ignore
 */

const handleUseMongoClient = function handleUseMongoClient(options) {
  console.warn('WARNING: The `useMongoClient` option is no longer ' +
    'necessary in mongoose 5.x, please remove it.');
  const stack = new Error().stack;
  console.warn(stack.substr(stack.indexOf('\n') + 1));
  delete options.useMongoClient;
};

/**
 * Closes the connection
 *
 * @param {Boolean} [force] optional
 * @param {Function} [callback] optional
 * @return {Promise}
 * @api public
 */

Connection.prototype.close = function(force, callback) {
  if (typeof force === 'function') {
    callback = force;
    force = false;
  }

  this.$wasForceClosed = !!force;

  return promiseOrCallback(callback, cb => {
    this._close(force, cb);
  });
};

/**
 * Handles closing the connection
 *
 * @param {Boolean} force
 * @param {Function} callback
 * @api private
 */
Connection.prototype._close = function(force, callback) {
  const _this = this;
  this._closeCalled = true;

  switch (this.readyState) {
    case STATES.disconnected:
      callback();
      break;

    case STATES.connected:
      this.readyState = STATES.disconnecting;
      this.doClose(force, function(err) {
        if (err) {
          return callback(err);
        }
        _this.onClose(force);
        callback(null);
      });

      break;
    case STATES.connecting:
      this.once('open', function() {
        _this.close(callback);
      });
      break;

    case STATES.disconnecting:
      this.once('close', function() {
        callback();
      });
      break;
  }

  return this;
};

/**
 * Called when the connection closes
 *
 * @api private
 */

Connection.prototype.onClose = function(force) {
  this.readyState = STATES.disconnected;

  // avoid having the collection subscribe to our event emitter
  // to prevent 0.3 warning
  for (const i in this.collections) {
    if (utils.object.hasOwnProperty(this.collections, i)) {
      this.collections[i].onClose(force);
    }
  }

  this.emit('close', force);
};

/**
 * Retrieves a collection, creating it if not cached.
 *
 * Not typically needed by applications. Just talk to your collection through your model.
 *
 * @param {String} name of the collection
 * @param {Object} [options] optional collection options
 * @return {Collection} collection instance
 * @api public
 */

Connection.prototype.collection = function(name, options) {
  options = options ? utils.clone(options) : {};
  options.$wasForceClosed = this.$wasForceClosed;
  if (!(name in this.collections)) {
    this.collections[name] = new Collection(name, this, options);
  }
  return this.collections[name];
};

/**
 * Declares a plugin executed on all schemas you pass to `conn.model()`
 *
 * Equivalent to calling `.plugin(fn)` on each schema you create.
 *
 * ####Example:
 *     const db = mongoose.createConnection('mongodb://localhost:27017/mydb');
 *     db.plugin(() => console.log('Applied'));
 *     db.plugins.length; // 1
 *
 *     db.model('Test', new Schema({})); // Prints "Applied"
 *
 * @param {Function} fn plugin callback
 * @param {Object} [opts] optional options
 * @return {Connection} this
 * @see plugins ./plugins.html
 * @api public
 */

Connection.prototype.plugin = function(fn, opts) {
  this.plugins.push([fn, opts]);
  return this;
};

/**
 * Defines or retrieves a model.
 *
 *     var mongoose = require('mongoose');
 *     var db = mongoose.createConnection(..);
 *     db.model('Venue', new Schema(..));
 *     var Ticket = db.model('Ticket', new Schema(..));
 *     var Venue = db.model('Venue');
 *
 * _When no `collection` argument is passed, Mongoose produces a collection name by passing the model `name` to the [utils.toCollectionName](#utils_exports.toCollectionName) method. This method pluralizes the name. If you don't like this behavior, either pass a collection name or set your schemas collection name option._
 *
 * ####Example:
 *
 *     var schema = new Schema({ name: String }, { collection: 'actor' });
 *
 *     // or
 *
 *     schema.set('collection', 'actor');
 *
 *     // or
 *
 *     var collectionName = 'actor'
 *     var M = conn.model('Actor', schema, collectionName)
 *
 * @param {String|Function} name the model name or class extending Model
 * @param {Schema} [schema] a schema. necessary when defining a model
 * @param {String} [collection] name of mongodb collection (optional) if not given it will be induced from model name
 * @see Mongoose#model #index_Mongoose-model
 * @return {Model} The compiled model
 * @api public
 */

Connection.prototype.model = function(name, schema, collection) {
  if (!(this instanceof Connection)) {
    throw new MongooseError('`connection.model()` should not be run with ' +
      '`new`. If you are doing `new db.model(foo)(bar)`, use ' +
      '`db.model(foo)(bar)` instead');
  }

  let fn;
  if (typeof name === 'function') {
    fn = name;
    name = fn.name;
  }

  // collection name discovery
  if (typeof schema === 'string') {
    collection = schema;
    schema = false;
  }

  if (utils.isObject(schema) && !schema.instanceOfSchema) {
    schema = new Schema(schema);
  }
  if (schema && !schema.instanceOfSchema) {
    throw new Error('The 2nd parameter to `mongoose.model()` should be a ' +
      'schema or a POJO');
  }

  if (this.models[name] && !collection) {
    // model exists but we are not subclassing with custom collection
    if (schema && schema.instanceOfSchema && schema !== this.models[name].schema) {
      throw new MongooseError.OverwriteModelError(name);
    }
    return this.models[name];
  }

  const opts = { cache: false, connection: this };
  let model;

  if (schema && schema.instanceOfSchema) {
    applyPlugins(schema, this.plugins, null, '$connectionPluginsApplied');

    // compile a model
    model = this.base.model(fn || name, schema, collection, opts);

    // only the first model with this name is cached to allow
    // for one-offs with custom collection names etc.
    if (!this.models[name]) {
      this.models[name] = model;
    }

    // Errors handled internally, so safe to ignore error
    model.init(function $modelInitNoop() {});

    return model;
  }

  if (this.models[name] && collection) {
    // subclassing current model with alternate collection
    model = this.models[name];
    schema = model.prototype.schema;
    const sub = model.__subclass(this, schema, collection);
    // do not cache the sub model
    return sub;
  }

  // lookup model in mongoose module
  model = this.base.models[name];

  if (!model) {
    throw new MongooseError.MissingSchemaError(name);
  }

  if (this === model.prototype.db
      && (!collection || collection === model.collection.name)) {
    // model already uses this connection.

    // only the first model with this name is cached to allow
    // for one-offs with custom collection names etc.
    if (!this.models[name]) {
      this.models[name] = model;
    }

    return model;
  }
  this.models[name] = model.__subclass(this, schema, collection);
  return this.models[name];
};

/**
 * Removes the model named `name` from this connection, if it exists. You can
 * use this function to clean up any models you created in your tests to
 * prevent OverwriteModelErrors.
 *
 * ####Example:
 *
 *     conn.model('User', new Schema({ name: String }));
 *     console.log(conn.model('User')); // Model object
 *     conn.deleteModel('User');
 *     console.log(conn.model('User')); // undefined
 *
 *     // Usually useful in a Mocha `afterEach()` hook
 *     afterEach(function() {
 *       conn.deleteModel(/.+/); // Delete every model
 *     });
 *
 * @api public
 * @param {String|RegExp} name if string, the name of the model to remove. If regexp, removes all models whose name matches the regexp.
 * @return {Connection} this
 */

Connection.prototype.deleteModel = function(name) {
  if (typeof name === 'string') {
    const model = this.model(name);
    if (model == null) {
      return this;
    }
    const collectionName = model.collection.name;
    delete this.models[name];
    delete this.collections[collectionName];
    delete this.base.modelSchemas[name];
  } else if (name instanceof RegExp) {
    const pattern = name;
    const names = this.modelNames();
    for (const name of names) {
      if (pattern.test(name)) {
        this.deleteModel(name);
      }
    }
  } else {
    throw new Error('First parameter to `deleteModel()` must be a string ' +
      'or regexp, got "' + name + '"');
  }

  return this;
};

/**
 * Watches the entire underlying database for changes. Similar to
 * [`Model.watch()`](/docs/api/model.html#model_Model.watch).
 *
 * This function does **not** trigger any middleware. In particular, it
 * does **not** trigger aggregate middleware.
 *
 * The ChangeStream object is an event emitter that emits the following events:
 *
 * - 'change': A change occurred, see below example
 * - 'error': An unrecoverable error occurred. In particular, change streams currently error out if they lose connection to the replica set primary. Follow [this GitHub issue](https://github.com/Automattic/mongoose/issues/6799) for updates.
 * - 'end': Emitted if the underlying stream is closed
 * - 'close': Emitted if the underlying stream is closed
 *
 * ####Example:
 *
 *     const User = conn.model('User', new Schema({ name: String }));
 *
 *     const changeStream = conn.watch().on('change', data => console.log(data));
 *
 *     // Triggers a 'change' event on the change stream.
 *     await User.create({ name: 'test' });
 *
 * @api public
 * @param {Array} [pipeline]
 * @param {Object} [options] passed without changes to [the MongoDB driver's `Db#watch()` function](https://mongodb.github.io/node-mongodb-native/3.4/api/Db.html#watch)
 * @return {ChangeStream} mongoose-specific change stream wrapper, inherits from EventEmitter
 */

Connection.prototype.watch = function(pipeline, options) {
  const disconnectedError = new MongooseError('Connection ' + this.id +
    ' was disconnected when calling `watch()`');

  const changeStreamThunk = cb => {
    immediate(() => {
      if (this.readyState === STATES.connecting) {
        this.once('open', function() {
          const driverChangeStream = this.db.watch(pipeline, options);
          cb(null, driverChangeStream);
        });
      } else if (this.readyState === STATES.disconnected && this.db == null) {
        cb(disconnectedError);
      } else {
        const driverChangeStream = this.db.watch(pipeline, options);
        cb(null, driverChangeStream);
      }
    });
  };

  const changeStream = new ChangeStream(changeStreamThunk, pipeline, options);
  return changeStream;
};

/**
 * Returns an array of model names created on this connection.
 * @api public
 * @return {Array}
 */

Connection.prototype.modelNames = function() {
  return Object.keys(this.models);
};

/**
 * @brief Returns if the connection requires authentication after it is opened. Generally if a
 * username and password are both provided than authentication is needed, but in some cases a
 * password is not required.
 * @api private
 * @return {Boolean} true if the connection should be authenticated after it is opened, otherwise false.
 */
Connection.prototype.shouldAuthenticate = function() {
  return this.user != null &&
    (this.pass != null || this.authMechanismDoesNotRequirePassword());
};

/**
 * @brief Returns a boolean value that specifies if the current authentication mechanism needs a
 * password to authenticate according to the auth objects passed into the openUri methods.
 * @api private
 * @return {Boolean} true if the authentication mechanism specified in the options object requires
 *  a password, otherwise false.
 */
Connection.prototype.authMechanismDoesNotRequirePassword = function() {
  if (this.options && this.options.auth) {
    return noPasswordAuthMechanisms.indexOf(this.options.auth.authMechanism) >= 0;
  }
  return true;
};

/**
 * @brief Returns a boolean value that specifies if the provided objects object provides enough
 * data to authenticate with. Generally this is true if the username and password are both specified
 * but in some authentication methods, a password is not required for authentication so only a username
 * is required.
 * @param {Object} [options] the options object passed into the openUri methods.
 * @api private
 * @return {Boolean} true if the provided options object provides enough data to authenticate with,
 *   otherwise false.
 */
Connection.prototype.optionsProvideAuthenticationData = function(options) {
  return (options) &&
      (options.user) &&
      ((options.pass) || this.authMechanismDoesNotRequirePassword());
};

/**
 * Switches to a different database using the same connection pool.
 *
 * Returns a new connection object, with the new db.
 *
 * @method useDb
 * @memberOf Connection
 * @param {String} name The database name
 * @param {Object} [options]
 * @param {Boolean} [options.useCache=false] If true, cache results so calling `useDb()` multiple times with the same name only creates 1 connection object.
 * @return {Connection} New Connection Object
 * @api public
 */

/*!
 * Module exports.
 */

Connection.STATES = STATES;
module.exports = Connection;
