## new.html.erb

<% form_for(@thing) do |f| %>
  <p>
    <b>Select A Project:</b><br />
     <%= select_tag "thing[project_id]", options_from_collection_for_select(current_user.projects, :id, :name) %>
  </p>
  <p>
    <b>Select a Workflow State:</b>
    <%= render :partial => 'workflow_state_select', :object => current_user.projects[0].workflow.workflow_states %>

<%= observe_field 'thing[project_id]', :frequency => 0.5, :update => 'thing[workflow_state_id]', :url => workflow_states_path(), :with => "'project_id=' + Form.Element.serialize(thing[project_id])" %>  </p>


# .. etc

## _workflow_state_select.html.erb

<%= select_tag "thing[workflow_state_id]",
          options_from_collection_for_select(workflow_state_select, :id, :name) %>


## views/workflow_states/index.js.rjs

replace_html 'thing[workflow_state_id]', :partial => 'workflow_state_select', :object => @workflow_states


## routes.rb

  map.resources :projects do |projects|
    projects.resources :workflow_states, :name_prefix => "project_"
  end


## generated javascript:

 <script type="text/javascript">
//<![CDATA[
new Form.Element.Observer('thing[project_id]', 0.5, function(element, value) {new Ajax.Updater('thing[workflow_state_id]', '/projects/NULL/workflow_states', {asynchronous:true, evalScripts:true, parameters:value})})
//]]>
</script>