public function limitedCount()
    {
        if (version_compare(\MongoClient::VERSION, '1.0.7', '<')) {
            throw new FeatureNotSupportedException('Limit and skip not supported in ext-mongo versions prior to 1.0.7');
        }

        return (int) $this->collection
            ->getMongoCollection()
            ->count(
                $this->expression->toArray(),
                $this->limit,
                $this->skip
            );
    }