jQuery Grid Plugin - jqGrid

Hello all,

I use a search form for filtering my data. At this form I have two dropdown boxes - master(sections) and slave(categories).
I populate slave dropdown box with categories on event beforeSearch. All works fine until I change section and naturally repopulate slave dropdown box. In post statement id of category do not change(post shows previous category's id), in spite of slave dropdown box is populated with new categories. In result filtering is not correct (empty grid).

This is the script block(search fom):

    jQuery("#srccontents").filterGrid("list",{
    gridModel:true,
    gridNames:true,
    formtype:"vertical",
    enableSearch: false,
    enableClear: false,
    autosearch: true,
    beforeSearch :
        function() {
            var kodkategorii = $('#sg_KODKATEGORII').val();
            var kodrazdela = $('#sg_SECTION').val();
            var phpurl = '<?php echo site_url() ?>/handbooks/viborka/razdel_kategoria/'+kodrazdela+'/'+kodkategorii+'/0';

            $.getJSON(phpurl,function(json) {
                var reskategorii = $(json.kategorii).html();
                $('#sg_KODKATEGORII').html(reskategorii);
                });
            }        
    });

Excume for my English.

Any suggestion?

Thanks a lot

Valery