## router.rb
Merb::Router.prepare do
  resources :posts do |posts|
    posts.resources :comments
  end

  default_routes
end

## /views/posts/show.html.haml
# ...
# This form url becomes: /posts/:post_id/comments/new
# What controller and action are expected?
= form_for(@comment, :action => url(:new_post_comment, @post)) do
  %p= text_field :posted_by
  %p= text_area :body
  %p= submit "Add Comment"
# ...