Used assertRaisesMessage() in m2m_through_regress tests.

The "needs to have a value for field" messages are incorrect and
reference nonexistent fields since the commit in which they were
introduced (refs #18823).

tests/m2m_through_regress/tests.py

@@ -67,11 +67,21 @@ class M2MThroughTestCase(TestCase):
             self.roll.members.set([])
 
     def test_cannot_use_create_on_m2m_with_intermediary_model(self):
-        with self.assertRaises(AttributeError):
+        msg = (
+            "Cannot use create() on a ManyToManyField which specifies an "
+            "intermediary model. Use m2m_through_regress.Membership's "
+            "Manager instead."
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
             self.rock.members.create(name="Anne")
 
     def test_cannot_use_create_on_reverse_m2m_with_intermediary_model(self):
-        with self.assertRaises(AttributeError):
+        msg = (
+            "Cannot use create() on a ManyToManyField which specifies an "
+            "intermediary model. Use m2m_through_regress.Membership's "
+            "Manager instead."
+        )
+        with self.assertRaisesMessage(AttributeError, msg):
             self.bob.group_set.create(name="Funk")
 
     def test_retrieve_reverse_m2m_items_via_custom_id_intermediary(self):
@@ -203,14 +213,27 @@ class ToFieldThroughTests(TestCase):
             ["<Driver: Barney Gumble>", "<Driver: Ryan Briscoe>"]
         )
 
-    def test_add_null(self):
-        nullcar = Car.objects.create(make=None)
-        with self.assertRaises(ValueError):
-            nullcar.drivers._add_items('car', 'driver', self.unused_driver)
+    def test_m2m_relations_unsable_on_null_to_field(self):
+        nullcar = Car(make=None)
+        msg = (
+            '"<Car: None>" needs to have a value for field "car" before this '
+            'many-to-many relationship can be used.'
+        )
+        with self.assertRaisesMessage(ValueError, msg):
+            nullcar.drivers.all()
+
+    def test_m2m_relations_unsable_on_null_pk_obj(self):
+        msg = (
+            "'Car' instance needs to have a primary key value before a "
+            "many-to-many relationship can be used."
+        )
+        with self.assertRaisesMessage(ValueError, msg):
+            Car(make='Ford').drivers.all()
 
     def test_add_related_null(self):
         nulldriver = Driver.objects.create(name=None)
-        with self.assertRaises(ValueError):
+        msg = 'Cannot add "<Driver: None>": the value for field "driver" is None'
+        with self.assertRaisesMessage(ValueError, msg):
             self.car.drivers._add_items('car', 'driver', nulldriver)
 
     def test_add_reverse(self):
@@ -228,12 +251,17 @@ class ToFieldThroughTests(TestCase):
 
     def test_add_null_reverse(self):
         nullcar = Car.objects.create(make=None)
-        with self.assertRaises(ValueError):
+        msg = 'Cannot add "<Car: None>": the value for field "car" is None'
+        with self.assertRaisesMessage(ValueError, msg):
             self.driver.car_set._add_items('driver', 'car', nullcar)
 
     def test_add_null_reverse_related(self):
         nulldriver = Driver.objects.create(name=None)
-        with self.assertRaises(ValueError):
+        msg = (
+            '"<Driver: None>" needs to have a value for field "driver" before '
+            'this many-to-many relationship can be used.'
+        )
+        with self.assertRaisesMessage(ValueError, msg):
             nulldriver.car_set._add_items('driver', 'car', self.car)
 
     def test_remove(self):