class MongoDb extends \lithium\data\Source {

	/**
	 * The default host used to connect to the server.
	 */
	const DEFAULT_HOST = 'localhost';

	/**
	 * The default port used to connect to the server.
	 */
	const DEFAULT_PORT = 27017;

	/**
	 * The Mongo class instance.
	 *
	 * @var object
	 */
	public $server = null;

	/**
	 * The MongoDB object instance.
	 *
	 * @var object
	 */
	public $connection = null;

	/**
	 * Classes used by this class.
	 *
	 * @var array
	 */
	protected $_classes = [
		'entity'       => 'lithium\data\entity\Document',
		'set'          => 'lithium\data\collection\DocumentSet',
		'result'       => 'lithium\data\source\mongo_db\Result',
		'schema'       => 'lithium\data\source\mongo_db\Schema',
		'exporter'     => 'lithium\data\source\mongo_db\Exporter',
		'relationship' => 'lithium\data\model\Relationship',
		'server'       => 'MongoClient'
	];

	/**
	 * Map of typical SQL-like operators to their MongoDB equivalents.
	 *
	 * @var array Keys are SQL-like operators, value is the MongoDB equivalent.
	 */
	protected $_operators = [
		'<'   => '$lt',
		'>'   => '$gt',
		'<='  => '$lte',
		'>='  => '$gte',
		'!='  => ['single' => '$ne', 'multiple' => '$nin'],
		'<>'  => ['single' => '$ne', 'multiple' => '$nin'],
		'or'  => '$or',
		'||'  => '$or',
		'not' => '$not',
		'!'   => '$not',
		'and' => '$and',
		'&&'  => '$and',
		'nor' => '$nor'
	];

	/**
	 * List of comparison operators to use when performing boolean logic in a query.
	 *
	 * @var array
	 */
	protected $_boolean = ['&&', '||', 'and', '$and', 'or', '$or', 'nor', '$nor'];

	/**
	 * A closure or anonymous function which receives an instance of this class, a
	 * collection name and associated meta information, and returns an array defining the
	 * schema for an associated model, where the keys are field names, and the values are
	 * arrays defining the type information for each field. At a minimum, type arrays
	 * must contain a `'type'` key. For more information on schema definitions see the
	 * `$_schema` property of the `Model` class.
	 *
	 * This example shows how to implement a schema callback in your database connection
	 * configuration that fetches and returns the schema data. It defines an optional
	 * MongoDB convention in which the schema for each individual collection is stored
	 * in a `schemas` collection, where each document contains the name of a collection,
	 * along with a `'data'` key, which contains the schema for that collection.
	 *
	 * ```
	 * Connections::add('default', [
	 *  'type' => 'MongoDb',
	 *  'host' => 'localhost',
	 *  'database' => 'app',
	 *  'schema' => function($db, $collection, $meta) {
	 *      $result = $db->connection->schemas->findOne(compact('collection'));
	 *      return $result ? $result['data'] : [];
	 *  }
	 * ]);
	 * ```
	 *
	 * A complete schema defintion looks like:
	 * ```
	 * [
	 *     '_id'  => ['type' => 'id'],
	 *     'name' => ['type' => 'string', 'default' => 'Moe', 'null' => false],
	 *     'sign' => ['type' => 'string', 'default' => 'bar', 'null' => false],
	 *     'age'  => ['type' => 'integer', 'default' => 0, 'null' => false]
	 * ];
	 * ```
	 *
	 * The tyes in the schema map to database native type like this:
	 * ```
	 *  id      => MongoId
	 *  date    => MongoDate
	 *  regex   => MongoRegex
	 *  integer => integer
	 *  float   => float
	 *  boolean => boolean
	 *  code    => MongoCode
	 *  binary  => MongoBinData
	 * ```
	 *
	 * @see lithium\data\Model::$_schema
	 * @var Closure|null
	 */
	protected $_schema = null;

	/**
	 * List of configuration keys which will be automatically assigned to their corresponding
	 * protected class properties.
	 *
	 * @var array
	 */
	protected $_autoConfig = ['schema', 'classes' => 'merge'];

	/**
	 * With no parameter, checks to see if adapter's dependencies are installed. With a
	 * parameter, queries for a specific supported feature.
	 *
	 * A compatibility layer cannot be detected via `extension_loaded()`, thus we check
	 * for the existence of one of the legacy classes to determine if this adapter can be
	 * enabled at all.
	 *
	 * @param string $feature Test for support for a specific feature, i.e. `"transactions"` or
	 *               `"arrays"`.
	 * @return boolean Returns `true` if the particular feature (or if MongoDB) support is enabled,
	 *         otherwise `false`.
	 */
	public static function enabled($feature = null) {
		if (!$feature) {
			return class_exists('MongoClient');
		}
		$features = [
			'arrays' => true,
			'transactions' => false,
			'booleans' => true,
			'relationships' => true,
			'schema' => false,
			'sources' => true
		];
		return isset($features[$feature]) ? $features[$feature] : null;
	}

	/**
	 * Constructor.
	 *
	 * @see lithium\data\Connections::add()
	 * @see lithium\data\source\MongoDb::$_schema
	 * @link http://php.net/mongo.construct.php PHP Manual: Mongo::__construct()
	 * @param array $config Configuration options required to connect to the database, including:
	 *        - `'host'` _string|array_: A string in the form of `'<host>'`, `'<host>:<port>'` or
	 *          `':<port>'` indicating the host and/or port to connect to. When one or both are
	 *          not provided uses general server defaults (if possible retrieved from the client
	 *          implementation).
	 *          Use the array format for multiple hosts:
	 *          `array('167.221.1.5:11222', '167.221.1.6')`
	 *        - `'database'` _string_: The name of the database to connect to. Defaults to `null`.
	 *        - `'timeout'` _integer_: The number of milliseconds a connection attempt will wait
	 *          before timing out and throwing an exception. Defaults to `100`.
	 *        - `'schema'` _\Closure_: A closure or anonymous function which returns the schema
	 *          information for a model class. See the `$_schema` property for more information.
	 *        - `'gridPrefix'` _string_: The default prefix for MongoDB's `chunks` and `files`
	 *          collections. Defaults to `'fs'`.
	 *        - `'replicaSet'` _string_: See the documentation for `Mongo::__construct()`. Defaults
	 *          to `false`.
	 *        - `'readPreference'` _mixed_: May either be a single value such as Mongo::RP_NEAREST,
	 *          or an array containing a read preference and a tag set such as:
	 *          (Mongo::RP_SECONDARY_PREFERRED, ['dc' => 'east] See the documentation for
	 *          `Mongo::setReadPreference()`. Defaults to null.
	 *          Typically, these parameters are set in `Connections::add()`, when adding the
	 *          adapter to the list of active connections.
	 *
	 *        Disables auto-connect, which is by default enabled in `Source`. Instead before
	 *        each query execution the connection is checked and if needed (re-)established.
	 * @return void
	 */
	public function __construct(array $config = []) {
		if (class_exists($server = $this->_classes['server'], false)) {
			$defaultHost = $server::DEFAULT_HOST . ':' . $server::DEFAULT_PORT;
		} else {
			$defaultHost = static::DEFAULT_HOST . ':' . static::DEFAULT_PORT;
		}
		$defaults = [
			'host' => $defaultHost,
			'login'      => null,
			'password'   => null,
			'database'   => null,
			'timeout'    => 100,
			'replicaSet' => false,
			'schema'     => null,
			'gridPrefix' => 'fs',
			'w'          => 1,
			'wTimeoutMS' => 10000,
			'readPreference' => null,
			'autoConnect' => false,
			'dsn' => null
		];
		parent::__construct($config + $defaults);
	}

	/**
	 * Initializer. Adds operator handlers which will later allow to correctly cast any
	 * values. Constructs a DSN from configuration.
	 *
	 * @see lithium\data\source\MongoDb::$_operators
	 * @see lithium\data\source\MongoDb::_operators()
	 * @return void
	 */
	protected function _init() {
		$hosts = [];
		foreach ((array) $this->_config['host'] as $host) {
			$host = HostString::parse($host) + [
				'host' => static::DEFAULT_HOST,
				'port' => static::DEFAULT_PORT
			];
			$hosts[] = "{$host['host']}:{$host['port']}";
		}
		if ($this->_config['login']) {
			$this->_config['dsn'] = sprintf(
				'mongodb://%s:%s@%s/%s',
				$this->_config['login'],
				$this->_config['password'],
				implode(',', $hosts),
				$this->_config['database']
			);
		} else {
			$this->_config['dsn'] = sprintf(
				'mongodb://%s',
				implode(',', $hosts)
			);
		}
		parent::_init();

		$this->_operators += [
			'like' => function($key, $value) {
				return new MongoRegex($value);
			},
			'$exists' => function($key, $value) {
				return ['$exists' => (boolean) $value];
			},
			'$type' => function($key, $value) {
				return ['$type' => (integer) $value];
			},
			'$mod' => function($key, $value) {
				$value = (array) $value;
				return ['$mod' => [current($value), next($value) ?: 0]];
			},
			'$size' => function($key, $value) {
				return ['$size' => (integer) $value];
			},
			'$elemMatch' => function($operator, $values, $options = []) {
				$options += [
					'castOpts' => [],
					'field' => ''
				];
				$options['castOpts'] += ['pathKey' => $options['field']];
				$values = (array) $values;

				if (empty($options['castOpts']['schema'])) {
					return ['$elemMatch' => $values];
				}
				foreach ($values as $key => &$value) {
					$value = $options['castOpts']['schema']->cast(
						null, $key, $value, $options['castOpts']
					);
				}
				return ['$elemMatch' => $values];
			}
		];
	}

	/**
	 * Destructor. Ensures that the server connection is closed and resources are freed when
	 * the adapter instance is destroyed.
	 *
	 * @return void
	 */
	public function __destruct() {
		if ($this->_isConnected) {
			$this->disconnect();
		}
	}

	/**
	 * Configures a model class by overriding the default dependencies for `'set'` and
	 * `'entity'` , and sets the primary key to `'_id'`, in keeping with Mongo's conventions.
	 *
	 * @see lithium\data\Model::$_meta
	 * @see lithium\data\Model::$_classes
	 * @param string $class The fully-namespaced model class name to be configured.
	 * @return Returns an array containing keys `'classes'` and `'meta'`, which will be merged with
	 *         their respective properties in `Model`.
	 */
	public function configureClass($class) {
		return [
			'classes' => $this->_classes,
			'schema' => [],
			'meta' => ['key' => '_id', 'locked' => false]
		];
	}

	/**
	 * Connects to the Mongo server. Matches up parameters from the constructor to create a Mongo
	 * database connection.
	 *
	 * @see lithium\data\source\MongoDb::__construct()
	 * @link http://php.net/mongo.construct.php PHP Manual: Mongo::__construct()
	 * @return boolean Returns `true` the connection attempt was successful, otherwise `false`.
	 */
	public function connect() {
		$server = $this->_classes['server'];

		if ($this->server && $this->server->getConnections() && $this->connection) {
			return $this->_isConnected = true;
		}
		$this->_isConnected = false;

		$options = [
			'connect' => true,
			'connectTimeoutMS' => $this->_config['timeout'],
			'replicaSet' => $this->_config['replicaSet'],
		];

		try {
			$this->server = new $server($this->_config['dsn'], $options);

			if ($prefs = $this->_config['readPreference']) {
				$prefs = !is_array($prefs) ? [$prefs, []] : $prefs;
				$this->server->setReadPreference($prefs[0], $prefs[1]);
			}

			if ($this->connection = $this->server->{$this->_config['database']}) {
				$this->_isConnected = true;
			}
		} catch (Exception $e) {
			throw new NetworkException("Could not connect to the database.", 503, $e);
		}
		return $this->_isConnected;
	}

	/**
	 * Disconnect from the Mongo server.
	 *
	 * Don't call the Mongo->close() method. The driver documentation states this should not
	 * be necessary since it auto disconnects when out of scope.
	 * With version 1.2.7, when using replica sets, close() can cause a segmentation fault.
	 *
	 * @return boolean True
	 */
	public function disconnect() {
		if ($this->server && $this->server->getConnections()) {
			$this->_isConnected = false;
			unset($this->connection, $this->server);
		}
		return true;
	}

	/**
	 * Returns the list of collections in the currently-connected database.
	 *
	 * @param string $class The fully-name-spaced class name of the model object making the request.
	 * @return array Returns an array of objects to which models can connect.
	 */
	public function sources($class = null) {
		$this->_checkConnection();
		$conn = $this->connection;
		return array_map(function($col) { return $col->getName(); }, $conn->listCollections());
	}

	/**
	 * Gets the column 'schema' for a given MongoDB collection. Only returns a schema if the
	 * `'schema'` configuration flag has been set in the constructor.
	 *
	 * @see lithium\data\source\MongoDb::$_schema
	 * @param mixed $collection Specifies a collection name for which the schema should be queried.
	 * @param mixed $fields Any schema data pre-defined by the model.
	 * @param array $meta Any meta information pre-defined in the model.
	 * @return array Returns an associative array describing the given collection's schema.
	 */
	public function describe($collection, $fields = [], array $meta = []) {
		if (!$fields && ($func = $this->_schema)) {
			$fields = $func($this, $collection, $meta);
		}
		return Libraries::instance(null, 'schema', compact('fields'), $this->_classes);
	}

	/**
	 * Quotes identifiers.
	 *
	 * MongoDb does not need identifiers quoted, so this method simply returns the identifier.
	 *
	 * @param string $name The identifier to quote.
	 * @return string The quoted identifier.
	 */
	public function name($name) {
		return $name;
	}

	/**
	 * A method dispatcher that allows direct calls to native methods in PHP's `Mongo` object. Read
	 * more here: http://php.net/manual/class.mongo.php
	 *
	 * For example (assuming this instance is stored in `Connections` as `'mongo'`):
	 * ```
	 * // Manually repairs a MongoDB instance
	 * Connections::get('mongo')->repairDB($db); // returns null
	 * ```
	 *
	 * @param string $method The name of native method to call. See the link above for available
	 *        class methods.
	 * @param array $params A list of parameters to be passed to the native method.
	 * @return mixed The return value of the native method specified in `$method`.
	 */
	public function __call($method, $params) {
		if ((!$this->server) && !$this->connect()) {
			return null;
		}
		return call_user_func_array([&$this->server, $method], $params);
	}

	/**
	 * Determines if a given method can be called.
	 *
	 * @deprecated
	 * @param string $method Name of the method.
	 * @param boolean $internal Provide `true` to perform check from inside the
	 *                class/object. When `false` checks also for public visibility;
	 *                defaults to `false`.
	 * @return boolean Returns `true` if the method can be called, `false` otherwise.
	 */
	public function respondsTo($method, $internal = false) {
		$message  = '`' . __METHOD__ . '()` has been deprecated. ';
		$message .= 'Use `is_callable([$adapter->server, \'<method>\'])` instead.';
		trigger_error($message, E_USER_DEPRECATED);

		$childRespondsTo = is_object($this->server) && is_callable([$this->server, $method]);
		return parent::respondsTo($method, $internal) || $childRespondsTo;
	}

	/**
	 * Normally used in cases where the query is a raw string (as opposed to a `Query` object),
	 * to database must determine the correct column names from the result resource. Not
	 * applicable to this data source.
	 *
	 * @internal param mixed $query
	 * @internal param \lithium\data\source\resource $resource
	 * @internal param object $context
	 * @return array
	 */
	public function schema($query, $resource = null, $context = null) {
		return [];
	}

	/**
	 * Create new document
	 *
	 * @param string $query
	 * @param array $options
	 * @return boolean
	 * @filter
	 */
	public function create($query, array $options = []) {
		$this->_checkConnection();

		$defaults = [
			'w' => $this->_config['w'],
			'wTimeoutMS' => $this->_config['wTimeoutMS'],
			'fsync' => false
		];
		$options += $defaults;

		$params = compact('query', 'options');

		return Filters::run($this, __FUNCTION__, $params, function($params) {
			$exporter = $this->_classes['exporter'];
			$prefix = $this->_config['gridPrefix'];

			$query   = $params['query'];
			$options = $params['options'];

			$args    = $query->export($this, ['keys' => ['source', 'data']]);
			$data    = $exporter::get('create', $args['data']);
			$source  = $args['source'];

			if ($source === "{$prefix}.files" && isset($data['create']['file'])) {
				$result = ['ok' => true];
				$data['create']['_id'] = $this->_saveFile($data['create']);
			} else {
				$result = $this->connection->{$source}->insert($data['create'], $options);
				$result = $this->_ok($result);
			}

			if ($result === true || isset($result['ok']) && (boolean) $result['ok'] === true) {
				if ($query->entity()) {
					$query->entity()->sync($data['create']['_id']);
				}
				return true;
			}
			return false;
		});
	}

	protected function _saveFile($data) {
		$uploadKeys = ['name', 'type', 'tmp_name', 'error', 'size'];
		$grid = $this->connection->getGridFS($this->_config['gridPrefix']);
		$file = null;
		$method = null;

		switch (true) {
			case  (is_array($data['file']) && array_keys($data['file']) == $uploadKeys):
				if (!$data['file']['error'] && is_uploaded_file($data['file']['tmp_name'])) {
					$method = 'storeFile';
					$file = $data['file']['tmp_name'];
					$data['filename'] = $data['file']['name'];
				}
			break;
			case $data['file']:
				$method = 'storeBytes';
				$file = $data['file'];
			break;
		}

		if (!$method || !$file) {
			return;
		}

		if (isset($data['_id'])) {
			$data += (array) get_object_vars($grid->get($data['_id']));
			$grid->delete($data['_id']);
		}
		unset($data['file']);
		return $grid->{$method}($file, $data);
	}

	/**
	 * Read from document
	 *
	 * @param string $query
	 * @param array $options
	 * @return object
	 * @filter
	 */
	public function read($query, array $options = []) {
		$this->_checkConnection();

		$defaults = ['return' => 'resource'];
		$options += $defaults;

		$params = compact('query', 'options');

		return Filters::run($this, __FUNCTION__, $params, function($params) {
			$prefix = $this->_config['gridPrefix'];

			$query = $params['query'];
			$options = $params['options'];
			$args = $query->export($this);
			$source = $args['source'];
			$model = $query->model();

			if ($group = $args['group']) {
				$result = $this->_group($group, $args, $options);
				$config = ['class' => 'set', 'defaults' => false] + compact('query') + $result;
				return $model::create($config['data'], $config);
			}
			$collection = $this->connection->{$source};

			if ($source === "{$prefix}.files") {
				$collection = $this->connection->getGridFS($prefix);
			}
			$result = $collection->find($args['conditions'], $args['fields']);

			if ($query->calculate()) {
				return $result;
			}

			$resource = $result->sort($args['order'])->limit($args['limit'])->skip($args['offset']);
			$result = Libraries::instance(null, 'result', compact('resource'), $this->_classes);
			$config = compact('result', 'query') + ['class' => 'set', 'defaults' => false];
			$collection = $model::create([], $config);

			if (is_object($query) && $query->with()) {
				$model::embed($collection, $query->with());
			}
			return $collection;
		});
	}

	protected function _group($group, $args, $options) {
		$conditions = $args['conditions'];
		$group += ['$reduce' => $args['reduce'], 'initial' => $args['initial']];
		$command = ['group' => $group + ['ns' => $args['source'], 'cond' => $conditions]];

		$stats = $this->connection->command($command);
		$data = isset($stats['retval']) ? $stats['retval'] : null;
		unset($stats['retval']);
		return compact('data', 'stats');
	}

	/**
	 * Update document
	 *
	 * @param string $query
	 * @param array $options
	 * @return boolean
	 * @filter
	 */
	public function update($query, array $options = []) {
		$this->_checkConnection();

		$defaults = [
			'upsert' => false,
			'multiple' => true,
			'w' => $this->_config['w'],
			'wTimeoutMS' => $this->_config['wTimeoutMS'],
			'fsync' => false
		];
		$options += $defaults;

		$params = compact('query', 'options');

		return Filters::run($this, __FUNCTION__, $params, function($params) {
			$exporter = $this->_classes['exporter'];
			$prefix = $this->_config['gridPrefix'];

			$options = $params['options'];
			$query  = $params['query'];
			$args   = $query->export($this, ['keys' => ['conditions', 'source', 'data']]);
			$source = $args['source'];
			$data   = $args['data'];

			if ($query->entity()) {
				$data = $exporter::get('update', $data);
			}

			if ($source === "{$prefix}.files" && isset($data['update']['file'])) {
				$args['data']['_id'] = $this->_saveFile($data['update']);
			}
			$update = $query->entity() ? $exporter::toCommand($data) : $data;

			if (empty($update)) {
				return true;
			}
			if ($options['multiple'] && !preg_grep('/^\$/', array_keys($update))) {
				$update = ['$set' => $update];
			}
			$result = $this->connection->{$source}->update($args['conditions'], $update, $options);

			if ($this->_ok($result)) {
				$query->entity() ? $query->entity()->sync() : null;
				return true;
			}
			return false;
		});
	}

	/**
	 * Delete document
	 *
	 * @param string $query
	 * @param array $options
	 * @return boolean
	 * @filter
	 */
	public function delete($query, array $options = []) {
		$this->_checkConnection();

		$defaults = [
			'justOne' => false,
			'w' => $this->_config['w'],
			'wTimeoutMS' => $this->_config['wTimeoutMS'],
			'fsync' => false
		];
		$options = array_intersect_key($options + $defaults, $defaults);
		$params = compact('query', 'options');

		return Filters::run($this, __FUNCTION__, $params, function($params) {
			$prefix = $this->_config['gridPrefix'];

			$query = $params['query'];
			$options = $params['options'];
			$args = $query->export($this, ['keys' => ['source', 'conditions']]);
			$source = $args['source'];
			$conditions = $args['conditions'];

			if ($source === "{$prefix}.files") {
				$result = $this->_deleteFile($conditions);
			} else {
				$result = $this->connection->{$args['source']}->remove($conditions, $options);
				$result = $this->_ok($result);
			}
			if ($result && $query->entity()) {
				$query->entity()->sync(null, [], ['dematerialize' => true]);
			}
			return $result;
		});
	}

	protected function _deleteFile($conditions, $options = []) {
		$_config = $this->_config;
		$defaults = ['w' => $_config['w'], 'wTimeoutMS' => $_config['wTimeoutMS']];
		$options += $defaults;
		$prefix = $this->_config['gridPrefix'];
		return $this->connection->getGridFS($prefix)->remove($conditions, $options);
	}

	/**
	 * Parse a `MongoCollection::<insert|update|delete>()` response and
	 * return `true` on success.
	 *
	 * @return boolean
	 */
	protected function _ok($result) {
		if (is_bool($result)) {
			return $result;
		}
		return !isset($result['err']) || $result['err'] === null;
	}

	/**
	 * Executes calculation-related queries, such as those required for `count`.
	 *
	 * @param string $type Only accepts `count`.
	 * @param mixed $query The query to be executed.
	 * @param array $options Optional arguments for the `read()` query that will be executed
	 *        to obtain the calculation result.
	 * @return integer Result of the calculation.
	 */
	public function calculation($type, $query, array $options = []) {
		$query->calculate($type);

		switch ($type) {
			case 'count':
				return $this->read($query, $options)->count();
		}
	}

	/**
	 * Document relationships.
	 *
	 * @param string $class
	 * @param string $type Relationship type, e.g. `belongsTo`.
	 * @param string $name
	 * @param array $config
	 * @return array
	 */
	public function relationship($class, $type, $name, array $config = []) {
		$fieldName = $this->relationFieldName($type, $name);
		$config += compact('name', 'type', 'key', 'fieldName');
		$config['from'] = $class;

		return Libraries::instance(null, 'relationship', $config + [
			'strategy' => function($rel) use ($config, $class, $name, $type) {
				if (isset($config['key'])) {
					return [];
				}
				$link = null;
				$hasLink = isset($config['link']);

				$result = [];
				$to = $rel->to();
				$local = $class::key();
				$className = $class::meta('name');

				$keys = [
					[$class, $name],
					[$class, Inflector::singularize($name)],
					[$to, Inflector::singularize($className)],
					[$to, $className]
				];
				foreach ($keys as $map) {
					list($on, $key) = $map;
					$key = lcfirst(Inflector::camelize($key));

					if (!$on::hasField($key)) {
						continue;
					}
					$join = ($on === $class) ? [$key => $on::key()] : [$local => $key];
					$result['key'] = $join;

					if (isset($config['link'])) {
						return $result;
					}
					$fieldType = $on::schema()->type($key);

					if ($fieldType === 'id' || $fieldType === 'MongoId') {
						$isArray = $on::schema()->is('array', $key);
						$link = $isArray ? $rel::LINK_KEY_LIST : $rel::LINK_KEY;
						break;
					}
				}
				if (!$link && !$hasLink) {
					$link = ($type === "belongsTo") ? $rel::LINK_CONTAINED : $rel::LINK_EMBEDDED;
				}
				return $result + ($hasLink ? [] : compact('link'));
			}
		], $this->_classes);
	}

	/**
	 * Formats `group` clauses for MongoDB.
	 *
	 * @param string|array $group The group clause.
	 * @param object $context
	 * @return array Formatted `group` clause.
	 */
	public function group($group, $context) {
		if (!$group) {
			return;
		}
		if (is_string($group) && strpos($group, 'function') === 0) {
			return ['$keyf' => new MongoCode($group)];
		}
		$group = (array) $group;

		foreach ($group as $i => $field) {
			if (is_int($i)) {
				$group[$field] = true;
				unset($group[$i]);
			}
		}
		return ['key' => $group];
	}

	/**
	 * Maps incoming conditions with their corresponding MongoDB-native operators.
	 *
	 * @param array $conditions Array of conditions
	 * @param object $context Context with which this method was called; currently
	 *        inspects the return value of `$context->type()`.
	 * @return array Transformed conditions
	 */
	public function conditions($conditions, $context) {
		if (!$conditions) {
			return [];
		}
		if ($code = $this->_isMongoCode($conditions)) {
			return $code;
		}
		$schema = null;
		$model = null;

		if ($context) {
			$schema = $context->schema();
			$model = $context->model();
		}
		return $this->_conditions($conditions, $model, $schema, $context);
	}

	/**
	 * Protected helper method used to format conditions.
	 *
	 * @todo Catch Document/Array objects used in conditions and extract their values.
	 * @param array $conditions The conditions array to be processed.
	 * @param string $model The name of the model class used in the query.
	 * @param object $schema The object containing the schema definition.
	 * @param object $context The `Query` object.
	 * @return array Processed query conditions.
	 */
	protected function _conditions(array $conditions, $model, $schema, $context) {
		$ops = $this->_operators;
		$castOpts = [
			'first' => true, 'database' => $this, 'wrap' => false, 'asContent' => true
		];

		$cast = function($key, $value) use (&$schema, &$castOpts) {
			return $schema ? $schema->cast(null, $key, $value, $castOpts) : $value;
		};

		foreach ($conditions as $key => $value) {
			if (in_array($key, $this->_boolean)) {
				$operator = isset($ops[$key]) ? $ops[$key] : $key;

				foreach ($value as $i => $compare) {
					$value[$i] = $this->_conditions($compare, $model, $schema, $context);
				}
				unset($conditions[$key]);
				$conditions[$operator] = $value;
				continue;
			}
			if (is_object($value)) {
				continue;
			}
			if (!is_array($value)) {
				$conditions[$key] = $cast($key, $value);
				continue;
			}
			$current = key($value);

			if (!isset($ops[$current]) && $current[0] !== '$') {
				$conditions[$key] = ['$in' => $cast($key, $value)];
				continue;
			}
			$conditions[$key] = $this->_operators($key, $value, $schema);
		}
		return $conditions;
	}

	protected function _isMongoCode($conditions) {
		if (is_string($conditions)) {
			$conditions = new MongoCode($conditions);
		}
		if ($conditions instanceof MongoCode) {
			return ['$where' => $conditions];
		}
	}

	protected function _operators($field, $operators, $schema) {
		$castOpts = compact('schema');
		$castOpts += ['first' => true, 'database' => $this, 'wrap' => false];

		$cast = function($key, $value) use (&$schema, &$castOpts) {
			return $schema ? $schema->cast(null, $key, $value, $castOpts) : $value;
		};

		foreach ($operators as $key => $value) {
			if (!isset($this->_operators[$key])) {
				$operators[$key] = $cast($field, $value);
				continue;
			}
			$operator = $this->_operators[$key];

			if (is_array($operator)) {
				$operator = $operator[is_array($value) ? 'multiple' : 'single'];
			}
			if (is_callable($operator)) {
				return $operator($key, $value, compact('castOpts', 'field'));
			}
			unset($operators[$key]);
			$operators[$operator] = $cast($field, $value);
		}
		return $operators;
	}

	/**
	 * Return formatted identifiers for fields.
	 *
	 * MongoDB does nt require field identifer escaping; as a result, this method is not
	 * implemented.
	 *
	 * @param array $fields Fields to be parsed
	 * @param object $context
	 * @return array Parsed fields array
	 */
	public function fields($fields, $context) {
		return $fields ?: [];
	}

	/**
	 * Return formatted clause for limit.
	 *
	 * MongoDB doesn't require limit identifer formatting; as a result, this method is not
	 * implemented.
	 *
	 * @param mixed $limit The `limit` clause to be formatted.
	 * @param object $context The `Query` object instance.
	 * @return mixed Formatted `limit` clause.
	 */
	public function limit($limit, $context) {
		return $limit ?: 0;
	}

	/**
	 * Return formatted clause for order.
	 *
	 * @param mixed $order The `order` clause to be formatted
	 * @param object $context
	 * @return mixed Formatted `order` clause.
	 */
	public function order($order, $context) {
		if (!$order) {
			return [];
		}
		if (is_string($order)) {
			return [$order => 1];
		}
		if (!is_array($order)) {
			return [];
		}
		foreach ($order as $key => $value) {
			if (!is_string($key)) {
				unset($order[$key]);
				$order[$value] = 1;
				continue;
			}
			if (is_string($value)) {
				$order[$key] = strtolower($value) === 'asc' ? 1 : -1;
			}
		}
		return $order;
	}

	protected function _checkConnection() {
		if (!$this->_isConnected && !$this->connect()) {
			throw new NetworkException("Could not connect to the database.");
		}
	}

	/**
	 * Returns the field name of a relation name (camelBack).
	 *
	 * @param string The type of the relation.
	 * @param string The name of the relation.
	 * @return string
	 */
	public function relationFieldName($type, $name) {
		$fieldName = Inflector::camelize($name, false);
		if (preg_match('/Many$/', $type)) {
			$fieldName = Inflector::pluralize($fieldName);
		} else {
			$fieldName = Inflector::singularize($fieldName);
		}
		return $fieldName;
	}
}