Using Rails 2.3.5

To do things RESTfully in Rails and you have a model relationship like:

class Post < ActiveRecord::Base
has_many :comments

You can set up your routes like this:

map.resources :post, :has_many => :comments

This will generate URLs like:

/posts/2/comments      [ GET (index), PUT (new), UPDATE (edit)]
/posts/2/comments/10 [ GET (show)]

Then to construct your form for creating or editing comments, you can do this with form_for:
form_for [@post, @comment] do |f|

Then in your CommentsController::update action, you will automatically have params[:post_id] so that comments are associated with the correct post

By the way, if you need to create those restful URLs by hand using url_for, this is how you would specify the create one:

{ :controller => :comments,
:action => :create,
:post_id => }

Notice that the controller is 'comments', even though the URL created with start with /posts

see form_for (ActionView::Helpers::FormHelper) - APIdock