多态性问题has_many

Question

我有下表“参与”：

id: integer
coupon_id: integer
participant_type: string
participant_id: integer

我有以下型号：

class Participation < ActiveRecord::Base
  belongs_to :coupon

  belongs_to :participant, :polymorphic => true

  belongs_to :group, :class_name => "Group", 
                     :foreign_key => "participant_id"
  belongs_to :location, :class_name => "Location", 
                        :foreign_key => "participant_id"
end


class Coupon < ActiveRecord::Base
  has_many        :partipations, :as => :participant
  has_many        :groups, :through => :participations, :source => :group, 
                  :conditions => "participants_type = 'Group'"
  has_many        :locations, :through => :participations, :source => :location, 
                  :conditions => "participants_type = 'Location'"
end

这与文章here中所示相似，也与ActiveRecord文档here类似。当访问带有错误的Coupon.first.groups或Coupon.first.locations时，它失败了：

ActiveRecord::HasManyThroughAssociationNotFoundError: 
Could not find the association :participations in model Coupon

我尝试了其他一些变化，但没有运气。当然，访问Group.first.participations会出现“未定义的方法”错误。