Nested forms

conversation.rb

class Conversation < ActiveRecord::Base
  has_many :messages
  has_many :participants
  has_many :users, :through => :participants

  accepts_nested_attributes_for :messages
end

conversations_controller.rb

class ConversationsController < ApplicationController
  load_and_authorize_resource :through => :current_user
  respond_to :html, :json, :js

  def index
  end

  def new
    @conversation.messages.build
  end

  def create
    @conversation.save
    respond_with @conversation
  end
end

message.rb

class Message < ActiveRecord::Base
  belongs_to :conversation
  belongs_to :author, :class_name => 'User'

  validates_presence_of :conversation
  validates_presence_of :author
  validates_presence_of :content
end

new.html.erb

<script type="text/javascript">
  $(function() {
      $(".chzn-select").chosen(); $(".chzn-select-deselect").chosen({allow_single_deselect:true});
  });
</script>
<%= @conversation.errors.messages %>
<%= form_for @conversation, :validate => true do |f| %>
  <div><%= f.label :user_ids, 'Recipients' %></div>
  <div><%= f.collection_select(:user_ids, User.all, :id, :email, options ={}, :class =>"chzn-select", :multiple => 'multiple') %></div>
  <%= f.fields_for :messages do |fm| %>
    <div><%= fm.label :content %></div>
    <div><%= fm.text_area :content %></div>
  <% end %>
  <div><%= f.submit %></div>
<% end %>