old_contents: from django.test import TestCase
  
  from referral.tests.factories import CampaignFactory
  
  
  class CampaignTestCase(TestCase):
      def test_model(self):
          obj = CampaignFactory()
          self.assertTrue(obj.pk)
  
      def test_count_users(self):
          obj = CampaignFactory()
          self.assertEqual(obj.count_users(),0)
  
  
new_contents: from django.test import TestCase
  
  from referral.tests.factories import CampaignFactory, ReferrerFactory
  
  
  class CampaignTestCase(TestCase):
      def test_model(self):
          obj = CampaignFactory()
          self.assertTrue(obj.pk)
  
      def test_count_users(self):
          obj = CampaignFactory()
          self.assertEqual(obj.count_users(),0)
  
  class ReferrerTestCase(TestCase):
      def test_model(self):
          obj = ReferrerFactory()
          self.assertTrue(obj.pk)
  
subject: add basic ReferrerTestCase