Refs #27462 -- Added tests of pk_set in m2m_changed signal receivers for repeated add/remove calls.

tests/m2m_signals/tests.py

@@ -69,6 +69,34 @@ class ManyToManySignalsTest(TestCase):
             self.m2m_changed_signal_receiver, Car.default_parts.through
         )
 
+    def test_pk_set_on_repeated_add_remove(self):
+        """
+        m2m_changed is always fired, even for repeated calls to the same
+        method, but the behavior of pk_sets differs by action.
+
+        - For signals related to `add()`, only PKs that will actually be
+          inserted are sent.
+        - For `remove()` all PKs are sent, even if they will not affect the DB.
+        """
+        pk_sets_sent = []
+
+        def handler(signal, sender, **kwargs):
+            if kwargs['action'] in ['pre_add', 'pre_remove']:
+                pk_sets_sent.append(kwargs['pk_set'])
+
+        models.signals.m2m_changed.connect(handler, Car.default_parts.through)
+
+        self.vw.default_parts.add(self.wheelset)
+        self.vw.default_parts.add(self.wheelset)
+
+        self.vw.default_parts.remove(self.wheelset)
+        self.vw.default_parts.remove(self.wheelset)
+
+        expected_pk_sets = [{1}, set(), {1}, {1}]
+        self.assertEqual(pk_sets_sent, expected_pk_sets)
+
+        models.signals.m2m_changed.disconnect(handler, Car.default_parts.through)
+
     def test_m2m_relations_add_remove_clear(self):
         expected_messages = []