updating a select box based on another (Ruby on Rails)

I'm in need of a more complete example on how to update a select box based on the results of a second select box in Ruby on Rails. I asked about this already here. I've read through the feedback from that posting, but am not having any luck figuring开发者_如何学运维 this out, and I've been trying for hours. Anybody know of a better (and more complete) example?

This is generally handled in Javascript. I don't particularly enjoy coding Javascript, so what I do for this in my application is to use a form_observer (a Rails helper which uses the Prototype Javascript library to watch your form for input changes) and have update a DIV in the HTML containing the second select box, based on the results of an AJAX call. Since AJAX talks to my server, I can write arbitrarily complex logic in Ruby to render the new HTML.

Example code:

#goes in view
<%= Code to render the first list box. %>
<%= render :partial => 'second_list_box_partial', :locals => {:selected = insert_magic_here } %>

<%= observe_field(:first_list_box,
    :url => { :action => :second_box_ajax }),
    :frequency => 0.5,
    :update => :second_list_box_div,
    :with => %Q| 'value=' + $('first_list_box').value;   |
    %>

#goes in controller
 def second_box_ajax
   first_box_value = params[:value]
   #magic goes here
   @selected = #more magic
   render :partial => 'second_list_box_partial', :locals => {:selected => @selected}, :layout => false
 end

 #goes in partial
 <div id="second_list_box_div">
   Actual code to render list box goes here.
 </div>