Fix RevisionQuerySet.for_instance() when used with a non-specific instance

If the method was used with a base Page instance, it wouldn't return any
revisions because we would be filtering on the content_type FK using the
current model (the base Page model) instead of the specific model.

Filter on base_content_type and make use of
RevisionMixin.get_base_content_type() instead, which will
resolve to the base Page model (and the correct the most basic
non-abstract model for non-Page models with MTI).

Use the old logic if the instance's model does not use RevisionMixin for
some reason.

This logic is similar to WorkflowStateQuerySet.for_instance() and
TaskQuerySet.for_instance().

wagtail/models/__init__.py

@@ -2792,12 +2792,20 @@ class RevisionQuerySet(models.QuerySet):
         return self.exclude(self.page_revisions_q())
 
     def for_instance(self, instance):
-        return self.filter(
-            content_type=ContentType.objects.get_for_model(
-                instance, for_concrete_model=False
-            ),
-            object_id=str(instance.pk),
-        )
+        try:
+            # Use RevisionMixin.get_base_content_type() if available
+            return self.filter(
+                base_content_type=instance.get_base_content_type(),
+                object_id=str(instance.pk),
+            )
+        except AttributeError:
+            # Fallback to ContentType for the model
+            return self.filter(
+                content_type=ContentType.objects.get_for_model(
+                    instance, for_concrete_model=False
+                ),
+                object_id=str(instance.pk),
+            )
 
 
 class RevisionsManager(models.Manager.from_queryset(RevisionQuerySet)):

wagtail/tests/test_revision_model.py

@@ -70,6 +70,9 @@ class TestRevisableModel(TestCase):
         self.assertEqual(self.instance.get_base_content_type(), self.content_type)
         self.assertEqual(self.instance.get_content_type(), self.content_type)
 
+        # The for_instance() method should return the revision
+        self.assertEqual(Revision.objects.for_instance(self.instance).first(), revision)
+
     def test_content_type_with_inheritance(self):
         instance = RevisableGrandChildModel.objects.create(text="test")
         instance.text = "test updated"
@@ -87,6 +90,18 @@ class TestRevisableModel(TestCase):
         self.assertEqual(instance.get_base_content_type(), base_content_type)
         self.assertEqual(instance.get_content_type(), content_type)
 
+        # The for_instance() method should return the revision,
+        # whether we're using the specific instance
+        self.assertIsInstance(instance, RevisableModel)
+        self.assertIsInstance(instance, RevisableGrandChildModel)
+        self.assertEqual(Revision.objects.for_instance(instance).first(), revision)
+
+        # or the base instance
+        base_instance = RevisableModel.objects.get(pk=instance.pk)
+        self.assertIsInstance(base_instance, RevisableModel)
+        self.assertNotIsInstance(base_instance, RevisableGrandChildModel)
+        self.assertEqual(Revision.objects.for_instance(base_instance).first(), revision)
+
     def test_content_type_for_page_model(self):
         hello_page = self.create_page()
         hello_page.content = "Updated world"
@@ -104,6 +119,18 @@ class TestRevisableModel(TestCase):
         self.assertEqual(hello_page.get_base_content_type(), base_content_type)
         self.assertEqual(hello_page.get_content_type(), content_type)
 
+        # The for_instance() method should return the revision,
+        # whether we're using the specific instance
+        self.assertIsInstance(hello_page, SimplePage)
+        self.assertIsInstance(hello_page, Page)
+        self.assertEqual(Revision.objects.for_instance(hello_page).first(), revision)
+
+        # or the base instance
+        base_instance = Page.objects.get(pk=hello_page.pk)
+        self.assertIsInstance(base_instance, Page)
+        self.assertNotIsInstance(base_instance, SimplePage)
+        self.assertEqual(Revision.objects.for_instance(base_instance).first(), revision)
+
     def test_as_object(self):
         self.instance.text = "updated"
         self.instance.save_revision()