comment.rb

class Comment < ActiveRecord::Base
   attr_accessible :invitation_id, :comment
   belongs_to :invitation
end

comments_controller.rb

class CommentsController < ApplicationController
  # GET /comments
  # GET /comments.json
  def index
    @comments = Comment.all

    respond_to do |format|
      format.html # index.html.test.erb
      format.json { render :json => @comments }
    end
  end

  # GET /comments/1
  # GET /comments/1.json
  def show
    @comment = Comment.find(params[:id])

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

  # GET /comments/new
  # GET /comments/new.json
  def new
    @comment = Comment.new

    respond_to do |format|
      format.html # new.html.erb
      format.json { render :json => @comment }
    end
  end

  # GET /comments/1/edit
  def edit
    @comment = Comment.find(params[:id])
  end

  # POST /comments
  # POST /comments.json
  def create
    @comment = Comment.new(params[:comment])

    respond_to do |format|
      if @comment.save
        format.html { redirect_to @comment, :notice => 'Comment was successfully created.' }
        format.json { render :json => @comment, :status => :created, :location => @comment }
      else
        format.html { render :action => "new" }
        format.json { render :json => @comment.errors, :status => :unprocessable_entity }
      end
    end
  end

  # PUT /comments/1
  # PUT /comments/1.json
  def update
    @comment = Comment.find(params[:id])

    respond_to do |format|
      if @comment.update_attributes(params[:comment])
        format.html { redirect_to @comment, :notice => 'Comment was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render :action => "edit" }
        format.json { render :json => @comment.errors, :status => :unprocessable_entity }
      end
    end
  end

  # DELETE /comments/1
  # DELETE /comments/1.json
  def destroy
    @comment = Comment.find(params[:id])
    @comment.destroy

    respond_to do |format|
      format.html { redirect_to comments_url }
      format.json { head :no_content }
    end
  end
end

invitation.rb

class Invitation < ActiveRecord::Base

  has_many :comments, :dependent => :destroy
  accepts_nested_attributes_for :comments, :allow_destroy => true

end

invitations_controller.rb

class InvitationsController < ApplicationController

  respond_to :html, :json

  # GET /invitations/1
  # GET /invitations/1.json
  def show
    @invitation = Invitation.find(params[:id])
    @comment = Comment.new
    respond_with(@invitation)
  end
end

routes.rb

# Other routes
  resources :comments

  resources :invitations do
    member do
       post :comments
    end
  end

show.html.erb

<p>
  <b>Title:</b>
  <%= @invitation.title %>
</p>

<%= form_for [@invitation, @comment] do |f| %>
    <%= f.text_area :comment, :rows => 3 %>
    <%= f.submit "Submit" %>
<% end %>