Fix handling of generic foreign keys in reference index

Fix two bugs with the handling of generic foreign keys within `ReferenceIndex._extract_references_from_object`:

* null values were not considered
* content type ID was copied directly from the source field, unlike all other cases that translate to the base content type instead - lookups use the base content type, so these results were not returned.

Fixes #9615

wagtail/models/reference_index.py

@@ -265,10 +265,15 @@ class ReferenceIndex(models.Model):
                 if isinstance(field, GenericForeignKey):
                     ct_field = object._meta.get_field(field.ct_field)
                     fk_field = object._meta.get_field(field.fk_field)
+                    ct_value = ct_field.value_from_object(object)
+                    fk_value = fk_field.value_from_object(object)
+
+                    if ct_value is not None and fk_value is not None:
+                        model = ContentType.objects.get_for_id(ct_value).model_class()
+                        yield cls._get_base_content_type(model).id, str(
+                            fk_value
+                        ), field.name, field.name
 
-                    yield ct_field.value_from_object(object), str(
-                        fk_field.value_from_object(object)
-                    ), field.name, field.name
                     continue
 
                 if isinstance(field, GenericRel):

wagtail/test/testapp/migrations/0012_set_genericsnippetpage_content_type_blank.py

@@ -0,0 +1,30 @@
+# Generated by Django 4.1.2 on 2022-11-07 16:23
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("contenttypes", "0002_remove_content_type_name"),
+        ("tests", "0011_modelwithnullableparentalkey"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="genericsnippetpage",
+            name="snippet_content_type",
+            field=models.ForeignKey(
+                blank=True,
+                null=True,
+                on_delete=django.db.models.deletion.SET_NULL,
+                to="contenttypes.contenttype",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="genericsnippetpage",
+            name="snippet_object_id",
+            field=models.PositiveIntegerField(blank=True, null=True),
+        ),
+    ]

wagtail/test/testapp/models.py

@@ -1567,9 +1567,9 @@ class GenericSnippetPage(Page):
     """
 
     snippet_content_type = models.ForeignKey(
-        ContentType, on_delete=models.SET_NULL, null=True
+        ContentType, on_delete=models.SET_NULL, null=True, blank=True
     )
-    snippet_object_id = models.PositiveIntegerField(null=True)
+    snippet_object_id = models.PositiveIntegerField(null=True, blank=True)
     snippet_content_object = GenericForeignKey(
         "snippet_content_type", "snippet_object_id"
     )

wagtail/tests/test_reference_index.py

@@ -10,6 +10,7 @@ from wagtail.models import Page, ReferenceIndex
 from wagtail.test.testapp.models import (
     EventPage,
     EventPageCarouselItem,
+    GenericSnippetPage,
     ModelWithNullableParentalKey,
 )
 
@@ -179,6 +180,19 @@ class TestCreateOrUpdateForObject(TestCase):
         refs = ReferenceIndex.get_references_to(self.event_page)
         self.assertEqual(refs.count(), 0)
 
+    def test_generic_foreign_key(self):
+        page1 = GenericSnippetPage(
+            title="generic snippet page", snippet_content_object=self.event_page
+        )
+        self.root_page.add_child(instance=page1)
+        page2 = GenericSnippetPage(
+            title="generic snippet page", snippet_content_object=None
+        )
+        self.root_page.add_child(instance=page2)
+
+        refs = ReferenceIndex.get_references_to(self.event_page)
+        self.assertEqual(refs.count(), 1)
+
     def test_rebuild_references_index_no_verbosity(self):
         stdout = StringIO()
         management.call_command(