mongodb geonear数组长度> 0

I want to sort the results with loc array length > 0

$cursor = $this->mongodb->command(array(
           'geoNear' => "place",        
           'near' => $lonlat,             
           'spherical' => true,            
           'num' => 20,    
           'query' => array("loc->0" => array("$exists" => true))
       ));

   [0] => Array
        (
            [loc] => Array
                (
                    [type] => Point
                    [coordinates] => Array
                        (
                            [0] => 32.929092407227
                            [1] => 39.960571289062
                        )

                )

        )

    [1] => Array
        (
            [loc] => Array
                (
                )

        )

I want to sort the results with loc array length > 0

How do I do it?

I Need only not empty loc