Fixed #34612 -- Fixed QuerySet.only() crash on reverse relationships.

Regression in b3db6c8dcb.

Thanks Ian Cubitt for the report.

This also corrected test_inheritance_deferred2() test which was
previously properly defined and marked as an expected failure but was
then wrongly adjusted to mask the lack of support for per-alias
deferral that was fixed by #21204.

django/db/models/sql/query.py

@@ -779,7 +779,13 @@ class Query(BaseExpression):
         # Only include fields mentioned in the mask.
         for field_name, field_mask in mask.items():
             field = opts.get_field(field_name)
-            field_select_mask = select_mask.setdefault(field, {})
+            # Retrieve the actual field associated with reverse relationships
+            # as that's what is expected in the select mask.
+            if field in opts.related_objects:
+                field_key = field.field
+            else:
+                field_key = field
+            field_select_mask = select_mask.setdefault(field_key, {})
             if field_mask:
                 if not field.is_relation:
                     raise FieldError(next(iter(field_mask)))

docs/releases/4.2.2.txt

@@ -20,6 +20,9 @@ Bugfixes
   when passing a ``ManyToManyField`` or ``GenericForeignKey`` reference. While
   doing so is a no-op, it was allowed in older version (:ticket:`34570`).
 
+* Fixed a regression in Django 4.2 that caused a crash of ``QuerySet.only()``
+  when passing a reverse ``OneToOneField`` reference (:ticket:`34612`).
+
 * Fixed a bug in Django 4.2 where :option:`makemigrations --update` didn't
   respect the ``--name`` option (:ticket:`34568`).
 

tests/defer_regress/tests.py

@@ -178,6 +178,16 @@ class DeferRegressionTest(TestCase):
             self.assertEqual(i.one_to_one_item.name, "second")
         with self.assertNumQueries(1):
             self.assertEqual(i.value, 42)
+        with self.assertNumQueries(1):
+            i = Item.objects.select_related("one_to_one_item").only(
+                "name", "one_to_one_item__item"
+            )[0]
+            self.assertEqual(i.one_to_one_item.pk, o2o.pk)
+            self.assertEqual(i.name, "first")
+        with self.assertNumQueries(1):
+            self.assertEqual(i.one_to_one_item.name, "second")
+        with self.assertNumQueries(1):
+            self.assertEqual(i.value, 42)
 
     def test_defer_with_select_related(self):
         item1 = Item.objects.create(name="first", value=47)

tests/select_related_onetoone/tests.py

@@ -249,6 +249,9 @@ class ReverseSelectRelatedTestCase(TestCase):
             self.assertEqual(p.child1.name2, "n2")
         p = qs.get(name2="n2")
         with self.assertNumQueries(0):
+            self.assertEqual(p.child1.value, 1)
+            self.assertEqual(p.child1.child4.value4, 4)
+        with self.assertNumQueries(2):
             self.assertEqual(p.child1.name1, "n1")
             self.assertEqual(p.child1.child4.name1, "n1")