Polymorphic associations and forms


I've got an issue where I'm creating a form in my view where the user
can add a Sponsorship to an Event (m2m relationship using has_many
:through). A Sponsorship belongs to an event and also belongs to a
sponsor. The sponsor is a polymorphic relationship with either a Partner
or a Company. So I have these columns set up:



I created a view, much like in railscasts 3-part complex forms tutorial
where I can dynamically add and remove sponsorships for the event, and
there's a <select> field for choosing whether the sponsorship is for a
Partner or Company and then I have 2 more <select> fields, one with a
list of Partners and one with a list of Companies. I set up virtual
attributes in the Event model for :company_id and :partner_id and I
have logic in that model as well which wires everything up the best I
can (because of the way this is working, I couldn't figure out how to
make everything work like magic as in the railscast, but I did get that
working for a non-polymorphic relationship elsewhere in the project).

Everything saves fine, but when I render the view, the Company and
Partner <select> fields aren't set to the proper values. This is because
they're set to use :partner_id and :company_id rather than :sponsor_id.

Is there any way of doing this? anyone know where I'm going wrong?

some sample code:

## the partial =>
<div class="sponsor">
<% fields_for 'event[sponsor_attributes][]', sponsor do |sf| -%>
    <%= sf.select(:sponsor_type, @sponsor_types, { :prompt => 'Select a
type...' }, :index => nil )%>
      <%= label_tag 'Candidate:' %>
      <%= sf.select(:company_id, @companies, {:prompt => 'Choose a
company...' }, :index => nil ) %>
      <%= label_tag 'Partner: ' %>
      <%= sf.select(:partner_id, @partners, { :prompt => 'Choose a
partner...' }, :index => nil ) %>

    <% if sponsor.new_record? -%>
      <%= link_to_function '[-] Remove', "this.up('.sponsor').remove()"
    <% else -%>
      <%= link_to_function '[-] Remove', "mark_for_destroy(this,
'sponsor')" %>
      <%= sf.hidden_field :id, :index => nil %>
      <%= sf.hidden_field :should_destroy, :index => nil, :class =>
'should_destroy' %>
    <% end -%>
<% end -%>

## the Event model's important stuff (yes, I know... this is a mess) =>
  def sponsor_attributes=(sponsor_attributes)
    sponsor_attributes.each do |attrib|
      unless attrib[:sponsor_type].blank? # it's just an empty field...
        if attrib[:id].blank? # it's a new sponsorship!
          s = sponsorships.build()
        else # it's an existing sponsorship
          s = sponsorships.detect { |sp| sp.id == attrib[:id].to_i }

        s.sponsor_type = attrib[:sponsor_type]

        # load the correct sponsor_id
        case attrib[:sponsor_type]
        when 'Company'
          s.sponsor_id = attrib[:company_id]
        when 'Partner'
          s.sponsor_id = attrib[:partner_id]

If I missed anything, let me know.

Thanks in advance.