private function ensureChunksIndex()
{
foreach ($this->chunksCollection->listIndexes() as $index) {
if ($index->isUnique() && $index->getKey() === ['files_id' => 1, 'n' => 1]) {
return;
}
}
$this->chunksCollection->createIndex(['files_id' => 1, 'n' => 1], ['unique' => true]);
}