title: Problem with polymorphic association in Rails
body: <p>I am trying to follow <a href="http://railscasts.com/episodes/154-polymorphic-association" rel="nofollow noreferrer">Ryan Bates screencast</a> but have an error message. I did the following:</p>
  
  <p>1) Create table</p>
  
  <pre><code>class CreateComments &lt; ActiveRecord::Migration
    def self.up
      create_table :comments do |t|
        t.references :commentable, :polymorphic =&gt; true
  </code></pre>
  
  <p>2) Setup models</p>
  
  <pre><code>class Comment &lt; ActiveRecord::Base
    belongs_to :commentable, :polymorphic =&gt; true
  
  class Product &lt; ActiveRecord::Base
    has_and_belongs_to_many :categories
    has_many :comments, :as =&gt; :commentable
  
  class Category &lt; ActiveRecord::Base
    has_and_belongs_to_many :products
    has_many :comments, :as =&gt; :commentable
  </code></pre>
  
  <p>3) Change controller show action</p>
  
  <pre><code>class CategoriesController &lt; ApplicationController
    def show
      @category = Category.find_by_permalink(params[:id])
      @commentable = @category
      @comment = Comment.new(:commentable =&gt; @category)
    end
  </code></pre>
  
  <p>4) Add a form to template views/categories/show.html.erb</p>
  
  <pre><code>&lt;% form_for [@commentable, Comment.new] do |f| %&gt;
    &lt;p&gt;
      &lt;%= f.label :name %&gt;&lt;br /&gt;
      &lt;%= f.text_field :name %&gt;
    &lt;/p&gt;
    &lt;p&gt;
      &lt;%= f.label :content %&gt;&lt;br /&gt;
      &lt;%= f.text_area :content %&gt;
    &lt;/p&gt;
    &lt;p&gt;
      &lt;%= f.submit 'Submit' %&gt;
    &lt;/p&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>5) After that I get error message by accessing /categories/my-category-permalink</p>
  
  <pre><code>NoMethodError in Categories#show
  undefined method `category_comments_path' for #&lt;ActionView::Base:0x69a9254&gt;
  </code></pre>
  
  <p>Could you help me to understand what I did wrong?
  In the original screencast Ryan accesses comments by /categories/permalink/comments using nested associations, but I don't need that. I want to write comments directly from my polymorphic objects.
  Thanks</p>