用php实现mongodb 分组排序

$keys = array('type'=>1); //依据type字段分组
$initial = array("items" => array('CreateTime'=>0)); //带到reduce中初始化
$reduce = "function (obj, prev) { 
    if(obj.CreateTime > prev.items.CreateTime){  //按照CreateTime来排序
        prev.items = obj;
    }
    	    
}";

$cand = array('condition'=>array('status'=>1)); //查询条件

$collection->group($keys, $initial, $reduce, $cond);