private function ensureChunkIndexes()
{
$chunkCollection = $this->getChunkCollection();
$indexKey = ['files_id' => 1, 'n' => 1];
foreach ($chunkCollection->listIndexes() as $index) {
if (!empty($index['unique']) && $index['key'] === $indexKey) {
return;
}
}
$chunkCollection->createIndex($indexKey, ['unique' => true]);
}