php mongodb aggregate group query

php mongodb aggregate group query

    public function group()
    {
        $r = D('Charge')->getCollection()->aggregate([
            ['$match'=>['cat'=>'55d17590f7c6d19c1e000039']],
            ['$group' => ['_id' => '$cat', 'money' => ['$sum' => '$price'], 'oc' => ['$sum' => 1]]],
            ['$sort' => ['money' => -1]]
        ]);
        json_out($r);
    }