title: form_for with nested resources
body: <p>I have a two-part question about form_for and nested resources. Let's say I'm writing a blog engine and I want to relate a comment to an article. I've defined a nested resource as follows:</p>
  
  <pre><code>map.resources :articles do |articles|
      articles.resources :comments
  end
  </code></pre>
  
  <p>The comment form is in the show.html.erb view for articles, underneath the article itself, for instance like this:</p>
  
  <pre><code>&lt;%= render :partial =&gt; "articles/article" %&gt;
  &lt;% form_for([ :article, @comment]) do |f| %&gt;
      &lt;%= f.text_area :text %&gt;
      &lt;%= submit_tag "Submit" %&gt;
  &lt;%  end %&gt;
  </code></pre>
  
  <p>This gives an error, "Called id for nil, which would mistakenly etc." I've also tried</p>
  
  <pre><code>&lt;% form_for @article, @comment do |f| %&gt;
  </code></pre>
  
  <p>Which renders correctly but relates f.text_area to the article's 'text' field instead of the comment's, and presents the html for the article.text attribute in that text area. So I seem to have this wrong as well. What I want is a form whose 'submit' will call the create action on CommentsController, with an article_id in the params, for instance a post request to /articles/1/comments. </p>
  
  <p>The second part to my question is, what's the best way to create the comment instance to begin with? I'm creating a @comment in the show action of the ArticlesController, so a comment object will be in scope for the form_for helper. Then in the create action of the CommentsController, I create new @comment using the params passed in from the form_for. </p>
  
  <p>Thanks!</p>