my question is simple but yet I didn't find a clear example of doing that.
I'm trying to add optgroup for a select and fill it with options using jquery for some reason it's not working.
this is my code:

```
$('#ptype').change(function() {
        $.post(
                   "/sell/site/index/categories", 
                   {
                      'parent_id': $('#ptype').val()
                   }, 
                   function(response){
                       var categories = $.parseJSON(response);

                       $('#sub_category').empty();
                        $.each(categories, function (index) {

                            var optgroup = $('<optgroup/>');

                            $.each(this.children, function (index) {
                                optgroup.add($("<option></option>")
                                            .attr("value",index)
                                            .text(this)); 
                            });

                            $("#sub_category").append(optgroup);

                        });

                        $("#sub_category").multiselect('refresh'); //refresh the select here
                    }
                );
        });

```

Any help would be greatly appreciated!
Thanks!