在活动记录中进行多对多自引用关系的正确方法是什么？

Question

我有以下活动记录模型：

class Relationship < ActiveRecord::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  

尝试 user.followers 时出现错误。
ArgumentError：未知密钥：标识符
来自 /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'