private function getCollection($type) {
        if ($this->collections[$type] === null) {
            try {
                $this->collections[$type] = $this->getMongoClient()->selectCollection(
                    $this->params['databaseName'],
                    $type
                );
            } catch (MongoException $e) {
                throw new DatabaseException('Could not select collection', 500, $e);
            }
        }

        return $this->collections[$type];
    }