Setting a primary attribute via form radio buttons - ruby-on-rails

Question

So I'm having an issue setting a primary image for my Dress object via a form.
The form allows the user to edit the dress details and then add/remove images to the form (using nested_form) and for each of them set a label and assign a primary image.
Everything works so far except for setting the primary image via radio buttons.
Dress Model:
class Dress < ActiveRecord::Base
has_many :dress_images
has_one :primary_dress_image, :class_name => "DressImage", :conditions => { :is_primary => true }
accepts_nested_attributes_for :dress_images, :allow_destroy => true
validates :name, :presence => true, :length => { :maximum => 99 }
end
DressImage Model
class DressImage < ActiveRecord::Base
belongs_to :dress
# Same as:
# def self.primary
# where(:is_primary => true)
# end
scope :primary, where(:is_primary => true)
# clear old primary if:
# this is a new record
# this is existing and is_primary has been set to true
before_save :clear_primary,
:if => Proc.new{ |r| (r.new_record? && r.is_primary) || (r.is_primary_changed? && r.is_primary) }
validates :label, :presence => true, :length => { :maximum => 60 }
validates :caption, :length => { :maximum => 200 }
mount_uploader :image, ImageUploader
def clear_primary
DressImage.update_all( {:is_primary => false}, :dress_id => self.dress_id )
end
end
Dress edit form
<h1>Dress</h1>
<% #dress.errors.full_messages.each do |msg| %>
<li><%= msg %></li>
<% end %> <%#= f.label :name %>
<%= nested_form_for #dress, :as => :dress, :url => { :action => :update }, :html=>{ :multipart => true } do |f| %>
<%= f.label :name %>
<%= f.text_field :name %>
<%= f.label :description %>
<%= f.text_area :description %>
<%= f.fields_for :dress_images do |dress_image_form| %>
<div class="dress-image">
<%= image_tag dress_image_form.object.image_url(:thumb) %>
<%= dress_image_form.text_field :label %>
<%= dress_image_form.file_field :image %>
<div class="primary-image-radio">
<%= dress_image_form.label :is_primary, "Main Image" %>
<%= f.radio_button :primary_dress_image_id, dress_image_form.object.id %>
</div>
<p>
<%= dress_image_form.link_to_remove "Remove this attachment" %>
</p>
</div>
<% end %>
<%= f.link_to_add "Add Photo", :dress_images %>
<%= f.submit "Save Dress" %>
<% end %>
With this radio button, the primary_dress_image_id attribute is set on the Dress object, but #dress.primary_dress_image gives a different result to the ID.
If I change the radio button to <%= dress_image_form.radio_button :is_primary, true %> it works better but because the name of each radio button is different, they are not treated as the same group.
I'm new to rails so I might be missing something completely obvious or doing it all wrong.

Answer 1

Here's one solution.
Add a hidden input to each of your nested field groups. Use a radio_button_tag instead of radio_button to make sure they are in the same group:
<div class="dress-image">
<%= dress_image_form.hidden_field :is_primary, :class => 'primary-image' %>
...
<%= radio_button_tag "select_primary_image", true, dress_image_form.object.is_primary? %>
...
</div>
Then add some javascript to update the hidden field according to the radio button selection:
$("body").on "change", ".primary-image-radio input:radio" ->
$(#).closest(".dress-image").find(".primary-image").val( $(#).is(":checked") )
You might need to modify the code a little because it's just an untested quick example, but it should give you an idea.

Answer 2

So I ended up using this method: <%= dress_image_form.radio_button :is_primary, true %> and using jquery to deselect all the other radio buttons when one is clicked.
This seems like a bit of a hacky method to me - there must be a purely Rails way of doing this without having to resort to JS? Until I find a better one, I'm going to stick with this solution.