page.copy to use `exclude_fields_in_copy` child & parental m2m relations

- fixes #5099
- test_copy_page_with_excluded_parental_and_child_relations
- ensure the tests are wrapped in a try/finally so that the model is always reset back even if tests fail
- update page model tests

CHANGELOG.txt

@@ -44,6 +44,7 @@ Changelog
  * Fix: Mark up a few strings for translation (Luiz Boaretto)
  * Fix: Invalid focal_point attribute on image edit view (Michał (Quadric) Sieradzki)
  * Fix: No longer expose the `.delete()` method on the default Page.objects manager (Nick Smith)
+ * Fix: `exclude_fields_in_copy` on Page models will now work for for modelcluster parental / many to many relations (LB (Ben Johnston))
 
 
 2.8.1 (14.04.2020)

docs/releases/2.9.rst

@@ -62,6 +62,7 @@ Bug fixes
  * Mark up a few strings for translation (Luiz Boaretto)
  * Invalid focal_point attribute on image edit view (Michał (Quadric) Sieradzki)
  * No longer expose the ``.delete()`` method on the default Page.objects manager (Nick Smith)
+ * ``exclude_fields_in_copy`` on Page models will now work for for modelcluster parental / many to many relations (LB (Ben Johnston))
 
 
 Upgrade considerations

wagtail/core/models.py

@@ -1136,6 +1136,10 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
 
         # copy child m2m relations
         for related_field in get_all_child_m2m_relations(specific_self):
+            # Ignore explicitly excluded fields
+            if related_field.name in exclude_fields:
+                continue
+
             field = getattr(specific_self, related_field.name)
             if field and hasattr(field, 'all'):
                 values = field.all()
@@ -1180,6 +1184,9 @@ class Page(AbstractPage, index.Indexed, ClusterableModel, metaclass=PageBase):
 
             parental_key_name = child_relation.field.attname
             child_objects = getattr(specific_self, accessor_name, None)
+            # Ignore explicitly excluded fields
+            if accessor_name in exclude_fields:
+                continue
 
             if child_objects:
                 for child_object in child_objects.all():

wagtail/core/tests/test_page_model.py

@@ -1195,6 +1195,67 @@ class TestCopyPage(TestCase):
         # special_field is in the list to be excluded
         self.assertNotEqual(page.special_field, new_page.special_field)
 
+    def test_copy_page_with_excluded_parental_and_child_relations(self):
+        """Test that a page will be copied with parental and child relations removed if excluded."""
+
+        try:
+            # modify excluded fields for this test
+            EventPage.exclude_fields_in_copy = ['advert_placements', 'categories', 'signup_link']
+
+            # set up data
+            christmas_event = EventPage.objects.get(url_path='/home/events/christmas/')
+            summer_category = EventCategory.objects.create(name='Summer')
+            holiday_category = EventCategory.objects.create(name='Holidays')
+
+            # add URL (to test excluding a basic field)
+            christmas_event.signup_link = "https://christmas-is-awesome.com/rsvp"
+
+            # add parental many to many relations
+            christmas_event.categories = (summer_category, holiday_category)
+            christmas_event.save()
+
+            # Copy it
+            new_christmas_event = christmas_event.copy(
+                update_attrs={'title': "New christmas event", 'slug': 'new-christmas-event'}
+            )
+
+            # check that the signup_link was NOT copied
+            self.assertEqual(christmas_event.signup_link, "https://christmas-is-awesome.com/rsvp")
+            self.assertEqual(new_christmas_event.signup_link, '')
+
+            # check that original event is untouched
+            self.assertEqual(
+                christmas_event.categories.count(),
+                2,
+                "Child objects (parental many to many) defined on the superclass were removed from the original page"
+            )
+
+            # check that parental many to many are NOT copied
+            self.assertEqual(
+                new_christmas_event.categories.count(),
+                0,
+                "Child objects (parental many to many) were copied but should be excluded"
+            )
+
+            # check that child objects on original event were left untouched
+            self.assertEqual(
+                christmas_event.advert_placements.count(),
+                1,
+                "Child objects defined on the original superclass were edited when copied"
+            )
+
+            # check that child objects were NOT copied
+            self.assertEqual(
+                new_christmas_event.advert_placements.count(),
+                0,
+                "Child objects defined on the superclass were copied and should not be"
+            )
+
+        finally:
+            # reset excluded fields for future tests
+            EventPage.exclude_fields_in_copy = []
+
+
 
 class TestSubpageTypeBusinessRules(TestCase, WagtailTestUtils):
     def test_allowed_subpage_models(self):