Fixed #35729 -- Enabled natural key serialization opt-out for subclasses.

Refactored serialization logic to allow models inheriting a natural_key()
method (e.g. AbstractBaseUser) to explicitly opt out of natural key
serialization by returning an empty tuple from the method.

Thanks Jonas Dittrich for the report.

Co-authored-by: Jacob Walls <jacobtylerwalls@gmail.com>

2 files changed, 52 insertions, 0 deletions

diff --git a/tests/serializers/models/natural.py b/tests/serializers/models/natural.py
index cfc57b1127..f32d781a76 100644
--- a/tests/serializers/models/natural.py
+++ b/tests/serializers/models/natural.py
@@ -2,6 +2,7 @@
 
 import uuid
 
+from django.contrib.auth.base_user import AbstractBaseUser
 from django.db import models
 
 
@@ -74,3 +75,19 @@ class FKAsPKNoNaturalKey(models.Model):
 
     def natural_key(self):
         raise NotImplementedError("This method was not expected to be called.")
+
+
+class SubclassNaturalKeyOptOutUser(AbstractBaseUser):
+    email = models.EmailField(unique=False, null=True, blank=True)
+    USERNAME_FIELD = "email"
+
+    def natural_key(self):
+        return ()
+
+
+class PostToOptOutSubclassUser(models.Model):
+    author = models.ForeignKey(SubclassNaturalKeyOptOutUser, on_delete=models.CASCADE)
+    title = models.CharField(max_length=100)
+    subscribers = models.ManyToManyField(
+        SubclassNaturalKeyOptOutUser, related_name="subscribed_posts", blank=True
+    )
diff --git a/tests/serializers/test_natural.py b/tests/serializers/test_natural.py
index b5b35708c6..322abeb7fc 100644
--- a/tests/serializers/test_natural.py
+++ b/tests/serializers/test_natural.py
@@ -9,6 +9,8 @@ from .models import (
     NaturalKeyAnchor,
     NaturalKeyThing,
     NaturalPKWithDefault,
+    PostToOptOutSubclassUser,
+    SubclassNaturalKeyOptOutUser,
 )
 from .tests import register_tests
 
@@ -250,6 +252,34 @@ def fk_as_pk_natural_key_not_called(self, format):
         self.assertEqual(obj.object.pk, o1.pk)
 
 
+def natural_key_opt_out_test(self, format):
+    """
+    When a subclass of AbstractBaseUser opts out of natural key serialization
+    by returning an empty tuple, both FK and M2M relations serialize as
+    integer PKs and can be deserialized without error.
+    """
+    user1 = SubclassNaturalKeyOptOutUser.objects.create(email="user1@example.com")
+    user2 = SubclassNaturalKeyOptOutUser.objects.create(email="user2@example.com")
+
+    post = PostToOptOutSubclassUser.objects.create(
+        author=user1, title="Post 2 (Subclass Opt-out)"
+    )
+    post.subscribers.add(user1, user2)
+
+    user_data = serializers.serialize(format, [user1], use_natural_primary_keys=True)
+    post_data = serializers.serialize(format, [post], use_natural_foreign_keys=True)
+
+    list(serializers.deserialize(format, user_data))
+    deserialized_posts = list(serializers.deserialize(format, post_data))
+
+    post_obj = deserialized_posts[0].object
+    self.assertEqual(user1.email, post_obj.author.email)
+    self.assertEqual(
+        sorted([user1.email, user2.email]),
+        sorted(post_obj.subscribers.values_list("email", flat=True)),
+    )
+
+
 # Dynamically register tests for each serializer
 register_tests(
     NaturalKeySerializerTests,
@@ -284,3 +314,8 @@ register_tests(
     "test_%s_fk_as_pk_natural_key_not_called",
     fk_as_pk_natural_key_not_called,
 )
+register_tests(
+    NaturalKeySerializerTests,
+    "test_%s_natural_key_opt_out",
+    natural_key_opt_out_test,
+)