Simple question on inserting child data

after doing a bunch of reading I have finally sat down to create a
simple app to dirve home a lor of the concepts to which have been read.
So I figured a simple blog app is the best way to do it. The bad
thing is that I am already hung up when creating replies to a blog
posting.

I can't figure out how to insert the post_id value into the reply
table. When creating a reply the user is directed to a page via a url
simular to the following http://localhost:3000/main/reply/4, with the
value 4 being the post_id that is being replied to.

In the controller I have:

class MainController < ApplicationController

  def index
    #display a list of the posts
    @posts = Post.find(:all, :order => "id desc")
  end

  def reply
    #display the post that is being replied to
    @post = Post.find(params[:id])
    @reply = Reply.new
  end

  def post_reply
    @reply = Reply.find(params[:id])
    redirect_to :index
  end

end