mongodb aggregate doctrine

        //db.PageSerp.aggregate({$match: {page_id: "521cbb536803fade44000016"}}, {$group: {_id: "$keyword_id"}}, {$skip: 4}, {$limit: 2});
        /** @var Connection $conn */
        $conn = $this->get('doctrine_mongodb')->getConnection();
        /** @var \MongoClient $mongo */
        $mongo = $conn->getMongo();
        if(!$mongo){
            $conn->connect();
            $mongo = $conn->getMongo();
        }
        /** @var \MongoCollection $coll */
        $coll = $mongo->selectDB('nt-us')->selectCollection('PageSerp');
        $ops = array(
            array(
                '$match' => array('page_id' => '521cbb536803fade44000016'),
            ),
            array(
                '$group' => array('_id' => '$keyword_id'),
            ),
            array('$skip' => 2),
            array('$limit' => 2),
        );
        $res = $coll->aggregate($ops);