Generate unique reference in Paper factory

spec/factories/papers.rb

@@ -4,7 +4,7 @@ FactoryGirl.define do
   factory :paper do
     name         { Faker::Lorem.sentence }
     url          { Faker::Internet.url("ris.example.org") }
-    reference    "MyString"
+    sequence(:reference) { |n| FactoryHelper.reference(n) }
     body         "leipzig"
     published_at "2015-07-20 21:16:53"
     scraped_at   "2015-07-20 21:16:53"

spec/factory_helper.rb

@@ -20,4 +20,10 @@ module FactoryHelper
     PAPER_TYPES.sample
   end
 
+  REFERENCE = "A-%05i/16"
+
+  def self.reference(seq)
+    REFERENCE % seq
+  end
+
 end