[4.2.x] Fixed #34779 -- Avoided unnecessary selection of non-nullable m2m fields without natural keys during serialization.

By using `select_related(None)` instead of `select_related()`, the
unnecessary joins are completely avoided. Note that the current tests
already covers the change, when the field is not `null=True`.

Regression in f9936deed1ff13b20e18bd9ca2b0750b52706b6c.

Backport of 517d3bb4dd17e9c51690c98d747b86a0ed8b2fbf from main

1 files changed, 4 insertions, 1 deletions

diff --git a/django/core/serializers/python.py b/django/core/serializers/python.py
index 0dc504aa34..7ec894aa00 100644
--- a/django/core/serializers/python.py
+++ b/django/core/serializers/python.py
@@ -80,7 +80,10 @@ class Serializer(base.Serializer):
 
                 def queryset_iterator(obj, field):
                     return (
-                        getattr(obj, field.name).select_related().only("pk").iterator()
+                        getattr(obj, field.name)
+                        .select_related(None)
+                        .only("pk")
+                        .iterator()
                     )
 
             m2m_iter = getattr(obj, "_prefetched_objects_cache", {}).get(