Q:

form_for with nested resources

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:
map.resources :articles do |articles|
    articles.resources :comments
end

The comment form is in the show.html.erb view for articles, underneath the article itself, for instance like this:
<%= render :partial => "articles/article" %>
<% form_for([ :article, @comment]) do |f| %>
    <%= f.text_area :text %>
    <%= submit_tag "Submit" %>
<%  end %>

This gives an error, "Called id for nil, which would mistakenly etc." I've also tried
<% form_for @article, @comment do |f| %>

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. 
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. 
Thanks!

A:

Travis R is correct. (I wish I could upvote ya.) I just got this working myself. With these routes:
resources :articles do
  resources :comments
end

You get paths like:
/articles/42
/articles/42/comments/99

routed to controllers at
app/controllers/articles_controller.rb
app/controllers/comments_controller.rb

just as it says at http://guides.rubyonrails.org/routing.html#nested-resources, with no special namespaces.
But partials and forms become tricky. Note the square brackets:
<%= form_for [@article, @comment] do |f| %>

Most important, if you want a URI, you may need something like this:
article_comment_path(@article, @comment)

Alternatively:
[@article, @comment]

as described at http://edgeguides.rubyonrails.org/routing.html#creating-paths-and-urls-from-objects
For example, inside a collections partial with comment_item supplied for iteration,
<%= link_to "delete", article_comment_path(@article, comment_item),
      :method => :delete, :confirm => "Really?" %>

What jamuraa says may work in the context of Article, but it did not work for me in various other ways.
There is a lot of discussion related to nested resources, e.g. http://weblog.jamisbuck.org/2007/2/5/nesting-resources
Interestingly, I just learned that most people's unit-tests are not actually testing all paths. When people follow jamisbuck's suggestion, they end up with two ways to get at nested resources. Their unit-tests will generally get/post to the simplest:
# POST /comments
post :create, :comment => {:article_id=>42, ...}

In order to test the route that they may prefer, they need to do it this way:
# POST /articles/42/comments
post :create, :article_id => 42, :comment => {...}

I learned this because my unit-tests started failing when I switched from this:
resources :comments
resources :articles do
  resources :comments
end

to this:
resources :comments, :only => [:destroy, :show, :edit, :update]
resources :articles do
  resources :comments, :only => [:create, :index, :new]
end

I guess it's ok to have duplicate routes, and to miss a few unit-tests. (Why test? Because even if the user never sees the duplicates, your forms may refer to them, either implicitly or via named routes.) Still, to minimize needless duplication, I recommend this:
resources :comments
resources :articles do
  resources :comments, :only => [:create, :index, :new]
end

Sorry for the long answer. Not many people are aware of the subtleties, I think.

A:

Be sure to have both objects created in controller: @post and @comment for the post, eg:
@post = Post.find params[:post_id]
@comment = Comment.new(:post=>@post)

Then in view:
<%= form_for([@post, @comment]) do |f| %>

Be sure to explicitly define the array in the form_for, not just comma separated like you have above.

A:

You don't need to do special things in the form.  You just build the comment correctly in the show action: 
class ArticlesController < ActionController::Base
  ....
  def show
    @article = Article.find(params[:id])
    @new_comment = @article.comments.build
  end
  ....
end

and then make a form for it in the article view:
<% form_for @new_comment do |f| %>
   <%= f.text_area :text %>
   <%= f.submit "Post Comment" %>
<% end %>

by default, this comment will go to the create action of CommentsController, which you will then probably want to put redirect :back into so you're routed back to the Article page. 

