          """
          The default manager, "objects", doesn't exist, because a custom one
          was provided.
          """
          with self.assertRaises(AttributeError):
              Book.objects
  
      def test_filtering(self):
          """
          Custom managers respond to usual filtering methods
 
</s> remove         with self.assertRaises(AttributeError):
 </s> add         msg = "'CustomQuerySet' object has no attribute 'manager_only'"
        with self.assertRaisesMessage(AttributeError, msg): </s> remove                 with self.assertRaises(AttributeError):
 </s> add                 msg = "%r object has no attribute 'optout_public_method'" % manager.__class__.__name__
                with self.assertRaisesMessage(AttributeError, msg): </s> remove         with self.assertRaises(TemplateSyntaxError):
 </s> add         with self.assertRaisesMessage(TemplateSyntaxError, "Invalid filter: 'fil'"): </s> remove         with self.assertRaises(AttributeError):
 </s> add         with self.assertRaisesMessage(AttributeError, "'Employee' object has no attribute 'id'"): </s> remove         with self.assertRaises(ImproperlyConfigured):
 </s> add         msg = "'apps.apps.NotAConfig' isn't a subclass of AppConfig."
        with self.assertRaisesMessage(ImproperlyConfigured, msg):
        msg = "type object 'Book' has no attribute 'objects'"
        with self.assertRaisesMessage(AttributeError, msg):
          """
          The default manager, "objects", doesn't exist, because a custom one
          was provided.
          """
        msg = "type object 'Book' has no attribute 'objects'"
        with self.assertRaisesMessage(AttributeError, msg):
              Book.objects
  
      def test_filtering(self):
          """
          Custom managers respond to usual filtering methods
keep keep keep keep replace keep keep keep keep keep
 Used assertRaisesMessage() to test Django's error messages. 
tests/custom_managers/tests.py