## routes.rb
map.resources :posts do |post|
  post.resources :comments
end

## comments controller
def create
  @post = Post.find(params[:post_id])
  @comment = @post.comments.new(params[:comment])

  if @comment.save
    # ...
  else
    # ...
  end
end

## posts controller
def show
  @post = Post.find(...)
  @comment = Comment.new
end

## view [html_rails]
<% form_for([@post, @comment]) do |f| %>
  ...
<% end %>