From 19e0587ee596debf77540d6a08ccb6507e60b6a7 Mon Sep 17 00:00:00 2001
From: Mark Evans <mark@hellobaton.com>
Date: Sat, 3 Sep 2022 09:53:58 -0400
Subject: [PATCH] Fixed #33937 -- Optimized serialization of related m2m fields
 without natural keys.

---
 AUTHORS                                   |  1 +
 django/core/serializers/python.py         |  8 ++++++-
 django/core/serializers/xml_serializer.py |  8 ++++++-
 tests/serializers/tests.py                | 27 +++++++++++++++++++++++
 4 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/AUTHORS b/AUTHORS
index 8c2ab361d2..e3c54397ff 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -616,6 +616,7 @@ answer newbie questions, and generally made Django that much better:
     Mario Gonzalez <gonzalemario@gmail.com>
     Mariusz Felisiak <felisiak.mariusz@gmail.com>
     Mark Biggers <biggers@utsl.com>
+    Mark Evans <mark@meltdownlabs.com>
     Mark Gensler <mark.gensler@protonmail.com>
     mark@junklight.com
     Mark Lavin <markdlavin@gmail.com>
diff --git a/django/core/serializers/python.py b/django/core/serializers/python.py
index a3918bf9d2..fa3bce2948 100644
--- a/django/core/serializers/python.py
+++ b/django/core/serializers/python.py
@@ -70,14 +70,20 @@ class Serializer(base.Serializer):
                 def m2m_value(value):
                     return value.natural_key()
 
+                def queryset_iterator(obj, field):
+                    return getattr(obj, field.name).iterator()
+
             else:
 
                 def m2m_value(value):
                     return self._value_from_field(value, value._meta.pk)
 
+                def queryset_iterator(obj, field):
+                    return getattr(obj, field.name).only("pk").iterator()
+
             m2m_iter = getattr(obj, "_prefetched_objects_cache", {}).get(
                 field.name,
-                getattr(obj, field.name).iterator(),
+                queryset_iterator(obj, field),
             )
             self._current[field.name] = [m2m_value(related) for related in m2m_iter]
 
diff --git a/django/core/serializers/xml_serializer.py b/django/core/serializers/xml_serializer.py
index 8d3918cfaa..1d3269c41a 100644
--- a/django/core/serializers/xml_serializer.py
+++ b/django/core/serializers/xml_serializer.py
@@ -146,14 +146,20 @@ class Serializer(base.Serializer):
                         self.xml.endElement("natural")
                     self.xml.endElement("object")
 
+                def queryset_iterator(obj, field):
+                    return getattr(obj, field.name).iterator()
+
             else:
 
                 def handle_m2m(value):
                     self.xml.addQuickElement("object", attrs={"pk": str(value.pk)})
 
+                def queryset_iterator(obj, field):
+                    return getattr(obj, field.name).only("pk").iterator()
+
             m2m_iter = getattr(obj, "_prefetched_objects_cache", {}).get(
                 field.name,
-                getattr(obj, field.name).iterator(),
+                queryset_iterator(obj, field),
             )
             for relobj in m2m_iter:
                 handle_m2m(relobj)
diff --git a/tests/serializers/tests.py b/tests/serializers/tests.py
index 48899cb3fb..3cc937d58d 100644
--- a/tests/serializers/tests.py
+++ b/tests/serializers/tests.py
@@ -410,6 +410,33 @@ class SerializersTestBase:
         self.assertEqual(self._get_field_values(child_data, "parent_m2m"), [])
         self.assertEqual(self._get_field_values(child_data, "parent_data"), [])
 
+    def test_serialize_only_pk(self):
+        with self.assertNumQueries(5) as ctx:
+            serializers.serialize(
+                self.serializer_name,
+                Article.objects.all(),
+                use_natural_foreign_keys=False,
+            )
+
+        categories_sql = ctx[1]["sql"]
+        self.assertNotIn(connection.ops.quote_name("meta_data_id"), categories_sql)
+        meta_data_sql = ctx[2]["sql"]
+        self.assertNotIn(connection.ops.quote_name("kind"), meta_data_sql)
+
+    def test_serialize_no_only_pk_with_natural_keys(self):
+        with self.assertNumQueries(5) as ctx:
+            serializers.serialize(
+                self.serializer_name,
+                Article.objects.all(),
+                use_natural_foreign_keys=True,
+            )
+
+        categories_sql = ctx[1]["sql"]
+        self.assertNotIn(connection.ops.quote_name("meta_data_id"), categories_sql)
+        # CategoryMetaData has natural_key().
+        meta_data_sql = ctx[2]["sql"]
+        self.assertIn(connection.ops.quote_name("kind"), meta_data_sql)
+
 
 class SerializerAPITests(SimpleTestCase):
     def test_stream_class(self):