protected function __construct($collection_name)
 {
     Kit::ensureString($collection_name);
     try {
         $this->collectionName = $collection_name;
         $this->collection = Loader::loadMongoDB()->selectCollection($collection_name);
     } catch (Exception $e) {
         throw new UserException('Initializing collection failed.', $collection_name, $e);
     }
 }