ruby-on-rails - 多态 has_many 的问题

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

这与此处的文章中所指出的一样，也类似于此处的 ActiveRecord 文档。访问 Coupon.first.groups 或 Coupon.first.locations 时失败，错误如下：

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。