Unit testing a polymorphic association with fixtures in rails 4

I need to unit test a promotions model where every campaign has a URL. There is a polymorphic association between promotions and reference_link. the reference link fixture reference_link: linkable: fix_1 (PromoCode)

How do I convince rails that the promotions fixture does indeed have a URL belonging to it?

In promotions test helper

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