How do you make a form out of a polymorophic table?

I am trying to create a comment that could comment on other comments but are all derived from a single post.

What is especially troubling for me is trying to figure out how to make it so that this can all be achieved in the post show and not its edit or new. Is this archtecturally reasonable?

That way I can access it via Post.comments, or Comment.comments etc. or Comments.parent

My Models:

#comment.rb

  belongs_to  :post
  belongs_to  :parent, :class_name => 'Comment'
  has_many    :children, :class_name => 'Comment'

  validates_presence_of :text

#post.rb

  has_many                      :comments
  accepts_nested_attributes_for :comments

posts_controller

def show
  @post = Post.find(params[:开发者_开发百科id])


  respond_to do |format|
    format.html # show.html.erb
    format.xml  { render :xml => @post }
  end
end

routes.rb

resource :comments

I made my comment table have a :text and :post_id attribute. Though I don't think it needs a :post_id,

What should my form look like, where should it be?

Here's my awful attempt :

  - form_for @post do |f|
    - f.fields_for :comments do |c|
      = f.label 'Comments'
      = f.text_area :text
      = f.submit 'Submit'

But it seems unecessary to do that.

Two Key components to finishing this was this :

Because its in the show, I need to specify the URL to which this posts :

- form_for @post do |f|
  - f.fields_for :comments, @comment, :url => edit_post_path(@post) do |c|
    = c.label 'Comments'
    = c.text_area :text
    = c.submit 'Submit'

But I was confused because where there were no errors, there was no textfield!

That because my controller didn't mention one. So I added this to def show

@comment = Comment.new

Ta da, it works now.

Full Code

Controller:

  def show
    @post = Post.find(params[:id])
    @comment = Comment.new

    respond_to do |format|
      format.html # show.html.erb
      format.xml  { render :xml => @post }
    end
  end