Demo of accepts_nested_attributes_for

Rails 3.2.1

Setup

rails new accepts_nested_attributes_for-demo
cd accepts_nested_attributes_for-demo
bundle exec rails generate scaffold Post title:string content:text
bundle exec rails generate model Comment content:text post:references
bundle exec rake db:create db:migrate

Edit app/models/post.rb

class Post < ActiveRecord::Base
  has_many :comments, :dependent => :destroy
  accepts_nested_attributes_for :comments, allow_destroy: true
end

Continue setup…

bundle exec rake db:seed

Edit app/views/posts/_form.html.erb add nested comments fields in the post form

<%= f.fields_for :comments do |comment_form| %>
  <%= comment_form.text_area :content  %>
<% end %>

Start server

bundle exec rails server
open http://localhost:3000/posts

Then edit the post comments and submit the form.