what is the correct way of doing a many-to-many self-referencing relationship in active record?

I have the following active record models:

class Relationship < ActiveRec开发者_如何学Cord::Base  
  belongs_to :user  
  belongs_to :follower, :class_name => 'User', :foreign_key => 'follower_id'  
end  

class User < ActiveRecord::Base  
  has_many :relationships  
  has_many :followers, :through => :relationships  
end  

I get an error when I try user.followers.

ArgumentError: Unknown key(s): identifier

from /home/.rvm/gems/ruby-1.9.2-p180/gems/activesupport-3.0.7/lib /active_support/core_ext/hash/keys.rb:43:in `assert_valid_keys'