updating a form.select based whenever another form.select change [ajax]

I have the following form :

<% form_for :registration, @registration, :url => { :action => "create" } , :html => {:class => 'generalform'} do |form| %> <%= render :partial => "form", :object => form %> <%= form.submit "Register", :class => 'go' %>&nbsp;<%= link_to 'Cancel', registrations_path %> <% end %>

with this partial : (2 form select) <div class="set" id="site_fields" style="margin-bottom:10px;">    <div class="setbox">     <%= form.label :trainee %><br />     <%= form.select(:trainee_id, @trainees.map {|s| [s.name, s.id]}, {:include_blank => '- select a trainee -' }, :style => "width: 200px;", :onchange => remote_function( :method => "post", :with => "'trainee_id=' + value", :url => choose_registrations_path ) )%>   </div>    <div id="select_epath" class="setbox" style="margin-left:10px;">     <%= form.label :educational_path %><br />     <%= form.select(:epath_id, @epaths.map {|g| [g.kp_path_name, g.id]}, {:include_blank => '- select an educational path -' }, :style => "width:200px;" ) %>   </div>   <div class="clear"></div> </div>

whenever the first select change (trainee) an Ajax request is fired (remote_function) and the choose action in controller registrations is executed ... working well , based on the trainee _id , an array @epaths should be displayed in the second form.select..

I I put the second select in a partial, I loose the form

should I write ALL the form in a select a refresh it, or can I refresh only the 2 select ?

thanks for your help

erwin