public function getCollection()
    {
        if (!$this->collection) {
            // gridfs
            if ($this->isFile) {
                $this->collection = $this->getConnection()->getMongoDB()->getGridFS($this->collectionName);
            // normal
            } else {
                $this->collection = $this->getConnection()->getMongoDB()->selectCollection($this->collectionName);
            }
        }

        return $this->collection;
    }