[4.2.x] Fixed #34570 -- Silenced noop deferral of many-to-many and GFK.

While deferring many-to-many and GFK has no effect, the previous
implementation of QuerySet.defer() ignore them instead of crashing.

Regression in b3db6c8dcb.

Thanks Paco Martínez for the report.

Backport of 99e5dff737 from main

django/db/models/sql/query.py

@@ -725,7 +725,15 @@ class Query(BaseExpression):
                 field_select_mask = select_mask.setdefault((field_name, relation), {})
                 field = relation.field
             else:
-                field = opts.get_field(field_name).field
+                reverse_rel = opts.get_field(field_name)
+                # While virtual fields such as many-to-many and generic foreign
+                # keys cannot be effectively deferred we've historically
+                # allowed them to be passed to QuerySet.defer(). Ignore such
+                # field references until a layer of validation at mask
+                # alteration time will be implemented eventually.
+                if not hasattr(reverse_rel, "field"):
+                    continue
+                field = reverse_rel.field
                 field_select_mask = select_mask.setdefault(field, {})
             related_model = field.model._meta.concrete_model
             self._get_defer_select_mask(

docs/releases/4.2.2.txt

@@ -15,3 +15,7 @@ Bugfixes
 
 * Restored, following a regression in Django 4.2, ``get_prep_value()`` call in
   ``JSONField`` subclasses (:ticket:`34539`).
+
+* Fixed a regression in Django 4.2 that caused a crash of ``QuerySet.defer()``
+  when passing a ``ManyToManyField`` or ``GenericForeignKey`` reference. While
+  doing so is a no-op, it was allowed in older version (:ticket:`34570`).

tests/defer_regress/tests.py

@@ -296,6 +296,12 @@ class DeferRegressionTest(TestCase):
         with self.assertNumQueries(1):
             self.assertEqual(leaf.second_child.value, 64)
 
+    def test_defer_many_to_many_ignored(self):
+        location = Location.objects.create()
+        request = Request.objects.create(location=location)
+        with self.assertNumQueries(1):
+            self.assertEqual(Request.objects.defer("items").get(), request)
+
 
 class DeferDeletionSignalsTests(TestCase):
     senders = [Item, Proxy]