foo.js

  function reloadSubCategories (category, submenu) {
    if (category && submenu) {
      $.getScript('/admin/categories/'+ category +'/submenus/'+ submenu +'/subcategories');
    } else {
      $('#post_subcategory_id').replaceWith("<select name='post[subcategory_id]' id='post_subcategory_id'></select>" );
    }
  }

  $('#post_category_id').parent().on('change', '#post_category_id', function(e){
    var cat = this.value;
    $('#submenu_category_id').val(cat)

    $.getScript('/admin/categories/' + this.value + '/submenus', function(e){
      var submenu = $('#post_submenu_id').val();
      reloadSubCategories(cat, submenu);
    })
  });

  $('#post_submenu_id').parent().on('change', '#post_submenu_id', function(e){
    var cat = $('#post_category_id').val();
    var submenu = $('#post_submenu_id').val();
    $('#subcategory_submenu_id').val(submenu);

    reloadSubCategories(cat, this.value);
  });

  $('#submenu_category_id').parent().on('change', '#submenu_category_id', function(e){
    $('#new_submenu').attr('action', '/admin/categories/' + this.value + '/submenus')
  })
  $('#subcategory_submenu_id').parent().on('change', '#subcategory_submenu_id', function(e){
    var cat = $('#post_category_id').val();
    $('#new_subcategory').attr('action', '/admin/categories/' + cat + '/submenus/'+ this.value + '/subcategories')
  })