单元测试与Rails4中的fixture的多态关联

Question

我需要对每个活动都有URL的促销模型进行单元测试。促销和reference_link之间存在多态关联。参考链接夹具：

reference_link:
  linkable: 
    fix_1 (PromoCode)

如何让rails相信促销fixture确实有一个属于它的URL？

在促销测试助手中：

test "should have a URL associated with the promo code" do
  promo_code = promo_codes(:fix_1)
  promo_code.reference_link.url = nil
  assert_not promo_code.valid?
  promo_code2 = promo_codes(:fix_2)
  assert promo_code2.valid?
end

promocode.rb

class PromoCode < ActiveRecord::Base
  belongs_to :reward
  has_one :reference_link, as: :linkable, dependent: :destroy
  validates :email, presence: true
  validates :code, presence: true
end

reference_link.rb

class ReferenceLink < ActiveRecord::Base
  belongs_to :linkable, polymorphic: true,touch: true

  validates :link_name, presence: true
  validates :link_url, presence: true 
  validates_format_of :link_url, :with => /\A#{URI::regexp(['http',    'https'])}\z/
  validates_length_of :link_url,:link_name, :maximum => 255  
end