jquery_ajax_get_json_rails.js

$.ajax({
                url: '/categories.json',
                type: 'get',
                contentType: 'application/json;',
                success: function(msg,a) {
                  $('#product_category_id').empty();
                  options="";
                  $.each(msg, function(){
                    options+= '<option value="'+this.category.id+'">'+this.category.name+'</option>';
                  });
                  $('#product_category_id').append(options);
                },
                error: function (XMLHttpRequest, textStatus, errorThrown) {
                  // typically only one of textStatus or errorThrown 
                  // will have info
                  this; // the options for this ajax request
                }
});