protected function cleanupMongo()
    {
        try {
            $list = $this->dbh->listCollections();
        } catch (\MongoException $e) {
            throw new \Exception(sprintf('Failed to list collections of the DB: %s', $e->getMessage()));
        }
        foreach ($list as $collection) {
            try {
                $collection->drop();
            } catch (\MongoException $e) {
                throw new \Exception(sprintf('Failed to drop collection: %s', $e->getMessage()));
            }
        }
    }