public function getChunkCursor($refresh = false)
    {
        if ($refresh || $this->_chunkCursor === null) {
            $file = $this->getDocument();
            $this->_chunkCursor = $this->collection->getChunkCollection()->find(
                ['files_id' => $file['_id']],
                [],
                ['sort' => ['n' => 1]]
            );
        }
        return $this->_chunkCursor;
    }