[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 b3db6c8dcb5145f7d45eff517bcd96460475c879.

Thanks Paco Martínez for the report.

Backport of 99e5dff737cd20b12d060e4794e097063b61ec40 from main

1 files changed, 9 insertions, 1 deletions

diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index b347c6bf3f..b615d06f28 100644
--- a/django/db/models/sql/query.py
+++ b/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(