Polymorphic Association with Comments - Ruby on Rails

Question

I am having a bit of trouble getting these polymorphic associations to work completely. I followed this tutorial www.railscasts.com/episodes/154-polymorphic-association, but that seems to only work if I am path /controller/ID#/comments when making a new post. If I try to render a partial comment form right on /controller/ID# I get this error when creating the comment:

undefined method `comments' for #<ActiveSupport::HashWithIndifferentAccess:0x10341b2d0>

I have three models:

class Comment < ActiveRecord::Base
    belongs_to :commentable, :polymorphic => true       
end


class Post < ActiveRecord::Base
    belongs_to :user
    has_many :comments, :as => :commentable
end

class Article < ActiveRecord::Base
    belongs_to :user
    has_many :comments, :as => :commentable
end

And this is my view for /articles/#{ID}

<%= @article.title %>
<%= @article.content %>
<%= @article.user.login %>
<%= link_to 'Edit Article', edit_article_path(@article) %>

<h2>Comments</h2>
<%= render "comments/comments" %>
<%= render "comments/comment" %>

And here is my comment partial:

<div class="post_comment">
<%= form_for [@commentable, Comment.new] do |f| %>
    <%= f.text_area :content %>
    <%= f.submit "Post" %>
<% end %>
</div>

Here is my create method in the comments controller:

def create
    @commentable = find_commentable
    @comment = @commentable.comments.build(params[:comment])
    @comment.user_id = current_user.id
    if @comment.save
        redirect_to :id => nil
    else    
        flash[:notice] = "something went wrong"
        redirect_to @commentable
    end 
end

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

I think I understand what the issue is, but not sure how to correct the problem. This form is looking for @commentable, but if the path isn't nested it cannot find @commentable (may be wrong).

Here are my routes:

devise_for :users do
        get "login", :to => "devise/sessions#new"
        get "register", :to => "devise/registrations#new"
    end

    resources :posts do
        resources :comments
        resources :tags
    end

    resources :articles do
        resources :comments
        resources :tags
    end

    resources :users do 
        resources :articles
        resources :comments
        resources :tags
        resources :posts
    end 

    resources :comments

    root :to => "home#index"

end