php yii2 MongoDB Aggregation Error “each item in the pipeline must be a document” : $collection=Yii::$app->mongodb->getCollection("lc_executive_allocate");          $result=$collection->aggregate(array(array('$match'=>array('stage'=>'2','completed'=>'1','status'=>'1','assignedOn'=>array('$gte'=>'2018-03-01','$lte'=>'2018-03-29'))),         array('$group'=>array('_id'=>array('userId'=>'$userId','count'=>array('$sum'=>'1')))),array('$sort'=>array('count'=>'-1')),array('$limit'=>'1'),'cursor'=>array()));