text: from djmodels.test import TestCase
  
  from .models import Article, Car, Driver, Reporter
  
  
  class ManyToOneNullTests(TestCase):
      def setUp(self):
          # Create a Reporter.
          self.r = Reporter(name='John Smith')
          self.r.save()
          # Create an Article.
          self.a = Article(headline="First", reporter=self.r)
          self.a.save()
          # Create an Article via the Reporter object.
          self.a2 = self.r.article_set.create(headline="Second")
          # Create an Article with no Reporter by passing "reporter=None".
          self.a3 = Article(headline="Third", reporter=None)
          self.a3.save()
          # Create another article and reporter
          self.r2 = Reporter(name='Paul Jones')
          self.r2.save()
          self.a4 = self.r2.article_set.create(headline='Fourth')
  
      def test_get_related(self):
          self.assertEqual(self.a.reporter.id, self.r.id)
          # Article objects have access to their related Reporter objects.
          r = self.a.reporter
          self.assertEqual(r.id, self.r.id)
  
      def test_created_via_related_set(self):
          self.assertEqual(self.a2.reporter.id, self.r.id)
  
      def test_related_set(self):
          # Reporter objects have access to their related Article objects.
          self.assertQuerysetEqual(self.r.article_set.all(), ['<Article: First>', '<Article: Second>'])
          self.assertQuerysetEqual(self.r.article_set.filter(headline__startswith='Fir'), ['<Article: First>'])
          self.assertEqual(self.r.article_set.count(), 2)
  
      def test_created_without_related(self):
          self.assertIsNone(self.a3.reporter)
          # Need to reget a3 to refresh the cache
          a3 = Article.objects.get(pk=self.a3.pk)
          with self.assertRaises(AttributeError):
              getattr(a3.reporter, 'id')
          # Accessing an article's 'reporter' attribute returns None
          # if the reporter is set to None.
          self.assertIsNone(a3.reporter)
          # To retrieve the articles with no reporters set, use "reporter__isnull=True".
          self.assertQuerysetEqual(Article.objects.filter(reporter__isnull=True), ['<Article: Third>'])
          # We can achieve the same thing by filtering for the case where the
          # reporter is None.
          self.assertQuerysetEqual(Article.objects.filter(reporter=None), ['<Article: Third>'])
          # Set the reporter for the Third article
          self.assertQuerysetEqual(self.r.article_set.all(), ['<Article: First>', '<Article: Second>'])
          self.r.article_set.add(a3)
          self.assertQuerysetEqual(
              self.r.article_set.all(),
              ['<Article: First>', '<Article: Second>', '<Article: Third>']
          )
          # Remove an article from the set, and check that it was removed.
          self.r.article_set.remove(a3)
          self.assertQuerysetEqual(self.r.article_set.all(), ['<Article: First>', '<Article: Second>'])
          self.assertQuerysetEqual(Article.objects.filter(reporter__isnull=True), ['<Article: Third>'])
  
      def test_remove_from_wrong_set(self):
          self.assertQuerysetEqual(self.r2.article_set.all(), ['<Article: Fourth>'])
          # Try to remove a4 from a set it does not belong to
          with self.assertRaises(Reporter.DoesNotExist):
              self.r.article_set.remove(self.a4)
          self.assertQuerysetEqual(self.r2.article_set.all(), ['<Article: Fourth>'])
  
      def test_set(self):
          # Use manager.set() to allocate ForeignKey. Null is legal, so existing
          # members of the set that are not in the assignment set are set to null.
          self.r2.article_set.set([self.a2, self.a3])
          self.assertQuerysetEqual(self.r2.article_set.all(), ['<Article: Second>', '<Article: Third>'])
          # Use manager.set(clear=True)
          self.r2.article_set.set([self.a3, self.a4], clear=True)
          self.assertQuerysetEqual(self.r2.article_set.all(), ['<Article: Fourth>', '<Article: Third>'])
          # Clear the rest of the set
          self.r2.article_set.set([])
          self.assertQuerysetEqual(self.r2.article_set.all(), [])
          self.assertQuerysetEqual(
              Article.objects.filter(reporter__isnull=True),
              ['<Article: Fourth>', '<Article: Second>', '<Article: Third>']
          )
  
      def test_set_clear_non_bulk(self):
          # 2 queries for clear(), 1 for add(), and 1 to select objects.
          with self.assertNumQueries(4):
              self.r.article_set.set([self.a], bulk=False, clear=True)
  
      def test_assign_clear_related_set(self):
          # Use descriptor assignment to allocate ForeignKey. Null is legal, so
          # existing members of the set that are not in the assignment set are
          # set to null.
          self.r2.article_set.set([self.a2, self.a3])
          self.assertQuerysetEqual(self.r2.article_set.all(), ['<Article: Second>', '<Article: Third>'])
          # Clear the rest of the set
          self.r.article_set.clear()
          self.assertQuerysetEqual(self.r.article_set.all(), [])
          self.assertQuerysetEqual(
              Article.objects.filter(reporter__isnull=True),
              ['<Article: First>', '<Article: Fourth>']
          )
  
      def test_assign_with_queryset(self):
          # Querysets used in reverse FK assignments are pre-evaluated
          # so their value isn't affected by the clearing operation in
          # RelatedManager.set() (#19816).
          self.r2.article_set.set([self.a2, self.a3])
  
          qs = self.r2.article_set.filter(headline="Second")
          self.r2.article_set.set(qs)
  
          self.assertEqual(1, self.r2.article_set.count())
          self.assertEqual(1, qs.count())
  
      def test_add_efficiency(self):
          r = Reporter.objects.create()
          articles = []
          for _ in range(3):
              articles.append(Article.objects.create())
          with self.assertNumQueries(1):
              r.article_set.add(*articles)
          self.assertEqual(r.article_set.count(), 3)
  
      def test_clear_efficiency(self):
          r = Reporter.objects.create()
          for _ in range(3):
              r.article_set.create()
          with self.assertNumQueries(1):
              r.article_set.clear()
          self.assertEqual(r.article_set.count(), 0)
  
      def test_related_null_to_field(self):
          c1 = Car.objects.create()
          d1 = Driver.objects.create()
          self.assertIs(d1.car, None)
          with self.assertNumQueries(0):
              self.assertEqual(list(c1.drivers.all()), [])
  