sources = new WebMarkupContainer("sources");
sources.setOutputMarkupId(true);
form.add(sources);
stores = new DropDownChoice<StoreInfo>("stores",
new Model(null), new StoreListModel(), new StoreListChoiceRenderer());
stores.add(new AjaxFormComponentUpdatingBehavior("onchange") {
@Override
protected void onUpdate(AjaxRequestTarget target) {
types.setChoices(new TypeListModel(stores.getModel()));