jquery sortable - gives empty array in response

Question

I am using jquery ui sortable for my codeigniter website but when i am sorting data it will give me empty array in console. please check and let me know my mistake.

    <script>
$(document).ready( function() {
    $("#sortme tbody").sortable({
        update : function (event, ui) {
        //    console.log(ui);
        serial = $('#sortme tbody td .ds').sortable();
            console.log(serial);

            $.ajax({
                url: "<?=base_url().'admin/pages/save_order'?>",
                type: "post",
                data: serial,
                success: function(response,data){
                    console.log(response);
                },
                error: function(){
                    alert("theres an error with AJAX");
                }
            });
         }
    });
});
</script>  

here controller

public function save_order()
{
    print_r($_POST);
    exit;
    $menu = $_POST['menu'];
    for ($i = 0; $i < count($menu); $i++)
    {
    echo $menu[$i];
    }
}

Answer 1

Use

var serial = $('#sortme tbody td .ds').sortable('serialize');