Fixed #36750 -- Made ordering of M2M objects deterministic in serializers.

Co-authored-by: Simon Charette <charette.s@gmail.com>
Co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

Author: 3 people

django/core/serializers/python.py

@@ -77,7 +77,15 @@ def queryset_iterator(obj, field):
                     chunk_size = (
                         2000 if getattr(attr, "prefetch_cache_name", None) else None
                     )
-                    return attr.iterator(chunk_size)
+                    query_set = attr.all()
+                    if not query_set.totally_ordered:
+                        current_ordering = (
+                            query_set.query.order_by
+                            or query_set.model._meta.ordering
+                            or []
+                        )
+                        query_set = query_set.order_by(*current_ordering, "pk")
+                    return query_set.iterator(chunk_size)
 
             else:
 
@@ -86,6 +94,13 @@ def m2m_value(value):
 
                 def queryset_iterator(obj, field):
                     query_set = getattr(obj, field.name).select_related(None).only("pk")
+                    if not query_set.totally_ordered:
+                        current_ordering = (
+                            query_set.query.order_by
+                            or query_set.model._meta.ordering
+                            or []
+                        )
+                        query_set = query_set.order_by(*current_ordering, "pk")
                     chunk_size = 2000 if query_set._prefetch_related_lookups else None
                     return query_set.iterator(chunk_size=chunk_size)
 

django/core/serializers/xml_serializer.py

@@ -177,7 +177,15 @@ def queryset_iterator(obj, field):
                     chunk_size = (
                         2000 if getattr(attr, "prefetch_cache_name", None) else None
                     )
-                    return attr.iterator(chunk_size)
+                    query_set = attr.all()
+                    if not query_set.totally_ordered:
+                        current_ordering = (
+                            query_set.query.order_by
+                            or query_set.model._meta.ordering
+                            or []
+                        )
+                        query_set = query_set.order_by(*current_ordering, "pk")
+                    return query_set.iterator(chunk_size)
 
             else:
 
@@ -186,6 +194,13 @@ def handle_m2m(value):
 
                 def queryset_iterator(obj, field):
                     query_set = getattr(obj, field.name).select_related(None).only("pk")
+                    if not query_set.totally_ordered:
+                        current_ordering = (
+                            query_set.query.order_by
+                            or query_set.model._meta.ordering
+                            or []
+                        )
+                        query_set = query_set.order_by(*current_ordering, "pk")
                     chunk_size = 2000 if query_set._prefetch_related_lookups else None
                     return query_set.iterator(chunk_size=chunk_size)
 

tests/serializers/test_natural.py

@@ -1,5 +1,7 @@
+from unittest import mock
+
 from django.core import serializers
-from django.db import connection
+from django.db import connection, models
 from django.test import TestCase
 
 from .models import (
@@ -336,6 +338,20 @@ def nullable_natural_key_m2m_test(self, format):
     )
 
 
+def natural_key_m2m_totally_ordered_test(self, format):
+    t1 = NaturalKeyThing.objects.create(key="t1")
+    t2 = NaturalKeyThing.objects.create(key="t2")
+    t3 = NaturalKeyThing.objects.create(key="t3")
+    t1.other_things.add(t2, t3)
+
+    with mock.patch.object(models.QuerySet, "order_by") as mock_order_by:
+        serializers.serialize(format, [t1], use_natural_foreign_keys=True)
+        mock_order_by.assert_called_once_with("pk")
+        mock_order_by.reset_mock()
+        serializers.serialize(format, [t1], use_natural_foreign_keys=False)
+        mock_order_by.assert_called_once_with("pk")
+
+
 # Dynamically register tests for each serializer
 register_tests(
     NaturalKeySerializerTests,
@@ -385,3 +401,8 @@ def nullable_natural_key_m2m_test(self, format):
     "test_%s_nullable_natural_key_m2m",
     nullable_natural_key_m2m_test,
 )
+register_tests(
+    NaturalKeySerializerTests,
+    "test_%s_natural_key_m2m_totally_ordered",
+    natural_key_m2m_totally_ordered_test,
+)