var base_url = '/';

function update_category() {

  var category_id = $('select1');

  var url = base_url + '/product/category_list.php';

  new Ajax.Updater( {success:"select2"},
                    url, {
                         method: "get",
                         parameters: "id=" + category_id.value,
                         onFailure: function() {alert("error");}
                         }
                   );

}