_dropdown.html.erb

<%- content_tag :li, :id => "dropdown_#{dropdown.id}" do %>

	<%= dropdown.name %>
	
	<%- unless dropdown.filtered_webpages.length.zero? -%>

		<ul class="sortable">

			<%= hidden_field_tag "webpages[dropdown_#{dropdown.id}]", :id => nil %>
			
			<%= render :partial => 'admin/dropdowns/webpage', :collection => dropdown.filtered_webpages %>

		</ul>

	<%- end -%>
	
<%- end -%>

_webpage.html.erb

<%= content_tag :li, webpage.name, :id => "webpage_#{webpage.id}" %>

dropdown.rb

class Dropdown < ActiveRecord::Base
  has_many :webpages, :order => "position", :dependent => :destroy  

  def self.update_positions(params)
    update_dropdown_positions(params[:dropdowns])
    update_webpage_positions(params[:webpages])
  end
  
  def self.update_dropdown_positions(dropdown_id_sequence)
    dropdown_id_sequence.split(",").each_with_index do |dropdown_id, i|
      Dropdown.find(dropdown_id).update_attribute(:position, i)
    end
  end
  
  def self.update_webpage_positions(webpage_params)
    webpage_params.values.each do |webpage_ids|
      webpage_ids.split(",").each_with_index do |webpage_id, i|
        Webpage.find(webpage_id).update_attribute(:position, i)
      end
    end
  end

end

dropdown_spec.rb

require 'spec_helper'
 
describe Dropdown do

  context 'updating positions' do
  
    def create_dropdowns
      @dropdowns = []
      
      3.times do
        @dropdowns << Dropdown.create(:name => 'some dropdown')
      end
      
      @dropdown_ids = @dropdowns.map(&:id)
    end
    
    def create_webpages
      
      3.times do
        @dropdowns.first.webpages << Webpage.create
      end
      
      @webpage_ids = @dropdowns.first.webpages.map(&:id)
    end
  
    before(:each) do
      create_dropdowns
      create_webpages
    end

    it 'should update the correct order of dropdowns' do
      dropdown_order = [@dropdown_ids[2], @dropdown_ids[0], @dropdown_ids[1]]
      Dropdown.update_dropdown_positions(dropdown_order.join(","))
      Dropdown.all(:order => :position).map(&:id).should == dropdown_order
    end
    
    it 'should update the correct order of webpages for a dropdown' do
       webpage_order = [@webpage_ids[2], @webpage_ids[0], @webpage_ids[1]]
       Dropdown.update_webpage_positions({"dropdown_#{@dropdowns.first.id}".to_sym => webpage_order.join(",")})
       @dropdowns.first.webpages.map(&:id).should == webpage_order
     end

  end

end

dropdowns_controller.rb

class Admin::DropdownsController < Admin::TablesController

  def reorder
    @dropdowns = Dropdown.all(:order => 'position')
  end
  
  def update_positions
    Dropdown.update_positions(params)
    
    flash[:notice] = "Successfully updated positions."
    redirect_to admin_dropdowns_url
  end
end

record_edit.js

Event.addBehavior({
	'.sortable': function() {
		Sortable.create(this, {'tag': 'li'});
	},
				
	'#update_dropdown_ordering:click': function() {
		$$('.sortable').each(function(e) {
			e.down('input').value = Sortable.sequence(e).join(",");
		});
	}
});

reorder.html.erb

<%- form_tag update_positions_admin_dropdowns_path, :method => 'put', :id => 'dropdown_position_form' do -%>
	
	<ul id="main_menu_list" class="sortable">

		<%= hidden_field_tag :dropdowns, :id => nil %>
			
		<%= render :partial => 'admin/dropdowns/dropdown', :collection => @dropdowns %>
	
	</ul>
	
	<div id="button_container">

		<%= link_to 'cancel', :back, :class => 'button_object' %>
		<%= submit_tag 'update', :class => 'button_object', :id => 'update_dropdown_ordering' %>

	</div>
	
<%- end -%>