Rails 3 nested resources, polymorphic associations and forms

Question

The question at hand deals with the key topics:

1. Nested Resources
2. Polymorphic Associations
3. Forms

We will assume we have Photos and Articles, both of which have Comments. This creates our Polymorphic Association.

#Photo Model -> photo.rb
    class Photo < ActiveRecord::Base
      has_many :comments, :as => :commentable
      accepts_nested_attributes_for :comments
    end

#Article Model -> article.rb
    class Article < ActiveRecord::Base
      has_many :comments, :as => :commentable
      accepts_nested_attributes_for :comments
    end

#Comment Model -> comment.rb
    class comment < ActiveRecord::Base
      belongs_to :commentable, :polymorphic => true
    end

Our View to add and view Comments is shared between Photos and Articles.

#Index View 
  <%= form_for [@comment] do |f| %>
     <%= render 'form', :f => f %>
     # more code...
  <% end %>

And, our resources Photos and Articles are nested in other resources as so:

# Routes.rb    
namespace :galleries do
   resources :photos do
     resources :comments
   end
end

namespace :blogs do
   resources :articles do
     resources :comments
   end
end

Now you can see in the form above, we have our polymorphic resource, but wee need our Parent resource and the Grand-Parent resource depending upon our request path. If hard coded (never gonna do it) we would have one of these two in the form_for:

<%= form_for [:galleries, :photos, @commetns] do |f| %>

or

<%= form_for [:blogs, :articles, @commetns] do |f| %>

Assuming we can find the parent from in our CommentsController as found in many articles and Stackoverflow answers like so:

def find_medical_loggable
  params.each do |name, value|
    if name =~ /(.+)_id$/
      return $1.classify.constantize.find(value)
    end
  end
end

How can we find the grand-parents and put all of this into the form_for helper?

Greatly appreciate it!