问Rails 4:未定义的方法`primary_key_name‘

EN

NoMethodError: undefined method `primary_key_name' for #<ActiveRecord::Reflection::AssociationReflection

class Store < ActiveRecord::Base
  has_many :relationships
  has_many :customers, :through => :relationships, :source => :user,
    :conditions => { :relationships => { :description => "Customer" } } do
      def <<(user)
        proxy_association.owner.relationships.create(:description => "Customer", :user => user)
      end
    end
end

class User < ActiveRecord::Base
  has_one :relationship
  has_one :store, :through => :relationship
end

class Relationship < ActiveRecord::Base
  belongs_to :store
  belongs_to :user
end