private function createCollection(DocumentManager $documentManager, ClassMetadata $classMetadata, $collectionName)
    {

        $database = $documentManager->getDatabase();

        $exists = false;
        foreach ($database->listCollections() as $collection) {
            if ($collection->getName() == $collectionName) {
                $exists = true;
            }
        }

        $creationOptions = $classMetadata->getCollectionCreationOptions();
        if (!empty($creationOptions) && !$exists) {
            $database->createCollection($collectionName, $creationOptions);
        }

        $collectionOptions = $classMetadata->getCollectionOptions();

        return $database->selectCollection($collectionName, $collectionOptions);
    }