Rails: Using accepts_nested_attributes_for in Nested Model Form

In my app, Users have many Conversations, and Conversations have many Messages. I want to create a new Conversation: I have to specify the Users (readers) and the (first) Message. I tried the following, but it failed.

Models

class Conversation < ActiveRecord::Base

  has_many :conversation_users
  has_many :users, :through => :conversation_users

  has_many :messages

  accepts_nested_attributes_for :users
  accepts_nested_attributes_for :messages

end

class Message < ActiveRecord::Base

  belongs_to :conversation
  belongs_to :user

end

class User < ActiveRecord::Base

  has_many :conversation_users
  has_many :conversations, :through => :conversation_users

end

Controller

def new
  @conversation = Conversation.new
  2.times do
    users = @conversation.users.build
  end
  messages = @conversation.messages.build
end

def create
  @conversation = Conversation.new(params[:conversation])

  if @conversation.save
    redirect_to username_conversations_path(current_usernam开发者_高级运维e)
  else
    redirect_to new_username_conversation_path(current_username)
  end
end

View

<% form_for([current_user, @conversation]) do |f| %>

   <% f.fields_for :users do |builder| %>

     <%= builder.text_field :id %>

   <% end %>

   <% f.fields_for :messages do |builder| %>

     <%= builder.text_area :content %>

   <% end %>

   <%= f.submit "Submit" %>

<% end %>

current_user and current_username are helper methods defined as follows:

def current_user
  @current_user ||= User.find(session[:user_id]) if session[:user_id]
end

def current_username
  @current_username ||= current_user.username if current_user
end

This is rails server's response:

Started POST "/users/8/conversations" for 127.0.0.1 at Sun Jul 17 23:58:27 +0200 2011
  Processing by ConversationsController#create as HTML
  Parameters: {"commit"=>"Submit", "authenticity_token"=>"z6kL+NmVspgCKMr9whcw+a85mA59j3jssS9QeTiEbxc=", "utf8"=>"✓", "conversation"=>{"users_attributes"=>{"0"=>{"id"=>"9"}, "1"=>{"id"=>"10"}}, "messages_attributes"=>{"0"=>{"content"=>"freee"}}}, "user_id"=>"8"}
  User Load (0.3ms)  SELECT "users".* FROM "users" INNER JOIN "conversation_users" ON "users".id = "conversation_users".user_id WHERE "users"."id" IN (9, 10) AND (("conversation_users".conversation_id = NULL))

ActiveRecord::RecordNotFound (Couldn't find User with ID=9 for Conversation with ID=):
  app/controllers/conversations_controller.rb:26:in `new'
  app/controllers/conversations_controller.rb:26:in `create'

line 26 of conversations_controller: @conversation = Conversation.new(params[:conversation])

How can I get this to work?

Thanks.

---

class Conversation < ActiveRecord::Base
  has_many :conversation_users
  has_many :users, :through => :conversation_users
  has_many :messages
  # NEXT LINE IS CHANGED!
  accepts_nested_attributes_for :conversation_users
  accepts_nested_attributes_for :messages
end

Controller

def new
  @conversation = Conversation.new
  2.times{ users = @conversation.conversation_users.build }
  messages = @conversation.messages.build
end

And form

<%= form_for([current_user, @conversation]) do |f| %>
  <%= f.fields_for :conversation_users do |builder| %>    
    <%= builder.text_field :user_id %>
    <%= builder.hidden_field :conversation_id %>
  <% end %>
  ...
<% end %>

That's it.

---

Replace:

   <% f.fields_for :users do |builder| %>

With:

   <%= f.fields_for :users do |builder| %>

Same missing '=' a bit lower.

---

<% form_for([current_user, @conversation]) do |f| %>

should be

<%= form_for([current_user, @conversation]) do |f| %>

And

<% f.fields_for :messages do |builder| %>
# and
<% f.fields_for :users do |builder| %>

should be

<%= f.fields_for :messages do |builder| %>
# and
<%= f.fields_for :users do |builder| %>