Rails belongs_to association (with :class_name) returns nil

Question

I'm relatively new to Rails development and I'm having a minor associations problem. I'd like to name an association something different than the model it's linked to.

I have the following 2 models:

class User < ActiveRecord::Base
  has_many :events
end

class Event < ActiveRecord::Base
  belongs_to :admin, :class_name => "User" # So we can call event.admin to retrieve the User who owns this Event
end

I build a User as follows:

event = event.create! :title => "New Event"

user = User.create! :username => "thinkswan"
user.events << event
user.save

When I hop into the console I receive the following:

irb> user = User.find(1)
irb> user.events
=> [#<Event id: 1, title: "New Event", user_id: 1, created_at: "2011-06-09 06:41:09", updated_at: "2011-06-09 06:41:10">]

irb> event = Event.find(1)
irb> event.user_id
=> 1
irb> event.admin
=> nil

Can anyone explain why the admin association isn't returning the User it's pointing to? Thanks!

+1 for including a good example in your question big man.. I finally understood what the `:class_name` option of `belongs_to` means.. the example in the [docs](http://guides.rubyonrails.org/v2.3.11/association_basics.html) wasn't quite clear — abbood, Apr 21 '13 at 14:43

score 49 · Accepted Answer · answered Jun 09 '11 at 15:26

49

You need to specify both :class_name and :foreign_key, for example:

belongs_to :admin, :class_name => "User", :foreign_key => "user_id"

answered Jun 09 '11 at 15:26

Jits

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I knew it would be a simple thing. Thanks so much, Jits! I appreciate it! – Graham Swan Jun 09 '11 at 15:44
No problem. One to watch out for in the future :) – Jits Jun 09 '11 at 15:45
cheers to you both, I finally understood that :admin is the variable name, and not the class_name ! – njzk2 Aug 13 '13 at 13:19
Is there a need to specify this in the migration? If so, how? – Eric T Mar 27 '14 at 17:10

Rails belongs_to association (with :class_name) returns nil

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