Question

I am developing a mentions system similar to the one in twitter. So I am dealing with three classes in this problem: Post, User and Mention. The mention object has a mentioned_id field (the mentioned user) and a post_id ( the post in which it happens).
I found some weird problems while writing tests, so I opened a console session, and this is what happens:
I create two users, two posts, and two mentions. If you notice, when I create the second mention (@mention2) the id of @mentioned_id is not the correct user’s id:

ruby-1.9.2-p180 :001 > @user1 = Factory(:user, :email => Factory.next(:email))

 => #<User id: 1, name: "Fulano", email: "[e-mail]", created_at: "2011-09-11 15:38:11", updated_at: "2011-09-11 15:38:11", encrypted_password: "ede4e8cc0440b8bd9fdc059e8496154715b6592690157aac618...", salt: "4bfae8fecee1629b7c290c2d5af5bd3bbeb38c4ce76546d7176...", admin: false> 

ruby-1.9.2-p180 :002 > @user2 = Factory(:user, :email => Factory.next(:email))

 => #<User id: 2, name: "Fulano", email: "[e-mail]", created_at: "2011-09-11 15:39:06", updated_at: "2011-09-11 15:39:06", encrypted_password: "ddda10f48575f63724e1db3d3cf684f2c60380325236311e15d...", salt: "911121b8942dc57116dfd24383d96874c750bc5a21fa210288b...", admin: false> 

ruby-1.9.2-p180 :003 > @post1 = Factory(:post, :user => @user1)

 => #<post id: 1, content: "Foo bar", user_id: 1, created_at: "2011-09-11 15:39:57", updated_at: "2011-09-11 15:39:57"> 

ruby-1.9.2-p180 :004 > @post2 = Factory(:post, :user => @user2)

 => #<post id: 2, content: "Foo bar", user_id: 2, created_at: "2011-09-11 15:40:37", updated_at: "2011-09-11 15:40:37"> 

ruby-1.9.2-p180 :005 > @mention1 = @post2.mentions.create :mentioned_id => @user1

 => #<Mention id: 1, post_id: 2, mentioned_id: 1, created_at: "2011-09-11 15:41:33", updated_at: "2011-09-11 15:41:33"> 

ruby-1.9.2-p180 :007 > @mention2 = @post1.mentions.create :mentioned_id => @user2

 => #<Mention id: 2, post_id: 1, mentioned_id: 1, created_at: "2011-09-11 15:42:16", updated_at: "2011-09-11 15:42:16"> 

Strangely enough, if I use @user2.id instead of just @user2 that last line, everything works fine:

ruby-1.9.2-p180 :008 > @mention2 = @post1.mentions.create :mentioned_id => @user2.id


=> #<Mention id: 3, post_id: 1, mentioned_id: 2, created_at: "2011-09-11 15:45:16", updated_at: "2011-09-11 15:45:16"> 

Can anyone please tell me what’s happening? I thought rails “magic” took care of getting the id when referencing an object. And why would it work with the first user but not with the second??

This is the code for my mention class:

class Mention < ActiveRecord::Base
  attr_accessible :mentioned_id

  belongs_to :mentioned, :class_name => "User"
  belongs_to :post

  validates :mentioned, :presence => true
  validates :post, :presence => true

end

And this is the relevant portion of the code for my user class:

    Class User < ActiveRecord::Base

  …

  has_many :posts, :dependent => :destroy  
  has_many :mentions, :dependent => :destroy, :foreign_key => 'mentioned_id'

  …

This also happens in my spec test:

 @user = Factory(:user, :email => Factory.next(:email))
  @other_user = Factory(:user, :email => Factory.next(:email))

  @post1 = Factory(:post, :user => @other_user)
  @post2 = Factory(:post, :user => @other_user)
  @post3 = Factory(:post, :user => @user) 
  @mention1 = @post1.mentions.create :mentioned_id => @user
  @mention2 = @post2.mentions.create :mentioned_id => @user 
  @mention3 = @post3.mentions.create :mentioned_id => @other_user.id

The last line only works correctly if I use the .id method, but the previous two work correctly…

Answer 1

You’re trying to ask Rails to map an integer (mentioned_id) to an object (@user). You’re doing it correctly on your @post1 = ... lines, where you’re assigning @other_user to :user, not :user_id. You need to do the same on your @mention1 = ... lines:

@mention1 = @post1.mentions.create :mentioned => @user

However, with your code as it is, this won’t work either, because your attr_accessor line is interfering.

You can either get rid of the line altogether and the :mentioned => @user part will work, or you can change it like so, to allow assigning directly to :mentioned rather than :mentioned_id (yes, they really are the same thing, but it seems that Rails checks attr_accessible and denies access to updating :mentioned before it realizes that :mentioned is really just :mentioned_id anyway).

attr_accessible :mentioned_id, :mentioned