public function getChunkCollection($refresh = false)
    {
        if ($refresh || !is_object($this->_chunkCollection)) {
            $this->_chunkCollection = Yii::createObject([
                'class' => 'yii\mongodb\Collection',
                'database' => $this->database,
                'name' => $this->getPrefix() . '.chunks'
            ]);
        }

        return $this->_chunkCollection;
    }