protected function storeItemInCache($key, CacheItemInterface $item, $ttl)
 {
     $object = ['_id' => $key, 'data' => serialize($item->get())];
     if ($ttl) {
         $object['expiresAt'] = new UTCDateTime((time() + $ttl) * 1000);
     }
     $this->collection->updateOne(['_id' => $key], ['$set' => $object], ['upsert' => true]);
     return true;
 }