Rails: radio button selection for nested forms objects

I have the following form for photo_album which uses the nested forms
feature of the Rails in saving the photos while updating the
photo_album. And having trouble with the selection of radio button
value. I want only one of the photos be able to select as the album
cover, but due to the way rails produces form element ids and names, I
am able to select all of the photos as album covers. Is there any
workaround?

<% form_for @photo_album do |f| %>
  <%= f.error_messages %>

  <% @photo_album.photos.each do |photo| %>
    <% f.fields_for :photos, photo do |photo_fields| %>
      <p>
       <%= image_tag url_for_image_column(photo, "data", :thumb) %>
      </p>
      <p>
       <%= photo_fields.label :title %>
       <%= photo_fields.text_field :title %>
      </p>
      <p>
       <%= photo_fields.label :body %>
       <%= photo_fields.text_area :body %>
      </p>
      <p>
       <%= photo_fields.radio_button :cover, "1" %>
       <%= photo_fields.label :cover, 'Album Cover', :class => 'option'
%>
       <%= photo_fields.check_box :_delete %>
       <%= photo_fields.label :_delete, 'Delete', :class => 'option' %>
      </p>
     <% end %>
   <% end %>

   <p>
     <%= f.submit @photo_album.new_record? ? 'Create' : 'Update' %>
    </p>
<% end %>

And following is the html produced by rails (which is part of the
problem) for radio buttons:

<p>
<input type="radio" value="1"
name="photo_album[photos_attributes][0][cover]"
id="photo_album_photos_attributes_0_cover_1"/>
<label for="photo_album_photos_attributes_0_cover" class="option">Album
Cover</label>
<input type="hidden" value="0"
name="photo_album[photos_attributes][0][_delete]"/><input
type="checkbox" value="1"
name="photo_album[photos_attributes][0][_delete]"
id="photo_album_photos_attributes_0__delete"/>
<label for="photo_album_photos_attributes_0__delete"
class="option">Delete</label>
</p>