Trying to understand Polymorphic Associations in Rails 3.2

[terenceponce]

I made the same question on StackOverflow.

I'm trying to understand Polymorphic Associations in Rails 3.2, but I'm having a hard time find good resources about it. I am trying to follow a RailsCasts episode about it, but it's pretty outdated, so I can't get it right. Here's what I've done so far:


I made a test application that has 2 independent models:


Foo

    class CreateFoos < ActiveRecord::Migration
      def change
        create_table :foos do |t|
          t.string :name
          t.text :content
    
          t.timestamps
        end
      end
    end

and Bar

    class CreateBars < ActiveRecord::Migration
      def change
        create_table :bars do |t|
          t.string :name
          t.text :content
    
          t.timestamps
        end
      end
    end

After that, I made the Comment model which will have a polymorphic association with Foo and Bar:

    class CreateComments < ActiveRecord::Migration
      def change
        create_table :comments do |t|
          t.text :content
          t.integer :commentable_id
          t.string :commentable_type
    
          t.timestamps
        end
      end
    end

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

Afterwards, I made the association to the other models:

    class Foo < ActiveRecord::Base
      has_many :comments, :as => :commentable
    end
    
    class Bar < ActiveRecord::Base
      has_many :comments, :as => :commentable
    end

I think I got the associations right. As for the routing, the routes.rb shown at the RailsCasts episode was pretty difference from what I was used to. I think this is because I started Rails at v3.0. Here's what I did:

    AnotherExerciseApp::Application.routes.draw do
      resources :foos do
        resources :comments
        resources :bars do
          resources :comments
        end
      end
    end

I'm not sure if this is the right way to do it, but it works. Following the RailsCasts episode, I made the method for finding which model the comment is associated with:

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

Afterwards, I changed the index action of the CommentsController to use the newly constructed method:

    def index
      @commentable = find_commentable
      @comments = @commentable.comments
    end

Here's where I went wrong: I have placed this code into the index page of the Comments view:

    <h1>Comments</h1>
    
    <ul id="comments">
      <% @comments.each do |comment| %>
        <li><%= comment.content %></li>
      <% end %>
    </ul>
    
    <h2>New Comment</h2>
    <% form_for [@commentable, Comment.new] do |form| %>
      <ol class="formList">
        <li>
          <%= form.label :content %>
          <%= form.text_area :content, :rows => 5 %>
        </li>
        <li><%= submit_tag "Add comment" %></li>
      </ol>
    <% end %>

For some reason, when I go to /foos/1/comments, the page is displaying, but the form isn't appearing. To be honest, I have no idea why that is the case. The only thing I noticed is that `form_for` is referencing to `@commentable`, which is the interface for the polymorphic association with the Comments model.

---------- Post added at 12:42 PM ---------- Previous post was at 12:21 PM ----------

As Katherine Pe has pointed out, the cause of the problem was a typo. I forgot to put an = on form_for. This is really embarrassing LOL. Kindly delete this shameful thread. Thanks!