Fixed #24607 -- Serialized natural keys in multi-table inheritance models.

Thanks João Paulo Melo de Sampaio for the test.

django/core/serializers/base.py

@@ -82,8 +82,16 @@ class Serializer:
             # Use the concrete parent class' _meta instead of the object's _meta
             # This is to avoid local_fields problems for proxy models. Refs #17717.
             concrete_model = obj._meta.concrete_model
+            # When using natural primary keys, retrieve the pk field of the
+            # parent for multi-table inheritance child models. That field must
+            # be serialized, otherwise deserialization isn't possible.
+            if self.use_natural_primary_keys:
+                pk = concrete_model._meta.pk
+                pk_parent = pk if pk.remote_field and pk.remote_field.parent_link else None
+            else:
+                pk_parent = None
             for field in concrete_model._meta.local_fields:
-                if field.serialize:
+                if field.serialize or field is pk_parent:
                     if field.remote_field is None:
                         if self.selected_fields is None or field.attname in self.selected_fields:
                             self.handle_field(obj, field)

tests/serializers/models/__init__.py

@@ -1,3 +1,4 @@
 from .base import *  # NOQA
 from .data import *  # NOQA
+from .multi_table import *  # NOQA
 from .natural import *  # NOQA

tests/serializers/models/multi_table.py

@@ -0,0 +1,19 @@
+from django.db import models
+
+
+class ParentManager(models.Manager):
+    def get_by_natural_key(self, parent_data):
+        return self.get(parent_data=parent_data)
+
+
+class Parent(models.Model):
+    parent_data = models.CharField(max_length=30, unique=True)
+
+    objects = ParentManager()
+
+    def natural_key(self):
+        return (self.parent_data,)
+
+
+class Child(Parent):
+    child_data = models.CharField(max_length=30, unique=True)

tests/serializers/test_natural.py

@@ -2,7 +2,7 @@ from django.core import serializers
 from django.db import connection
 from django.test import TestCase
 
-from .models import FKDataNaturalKey, NaturalKeyAnchor
+from .models import Child, FKDataNaturalKey, NaturalKeyAnchor
 from .tests import register_tests
 
 
@@ -67,6 +67,33 @@ def natural_key_test(self, format):
     self.assertIsNone(books[1].object.pk)
 
 
+def natural_pk_mti_test(self, format):
+    """
+    If serializing objects in a multi-table inheritance relationship using
+    natural primary keys, the natural foreign key for the parent is output in
+    the fields of the child so it's possible to relate the child to the parent
+    when deserializing.
+    """
+    child_1 = Child.objects.create(parent_data='1', child_data='1')
+    child_2 = Child.objects.create(parent_data='2', child_data='2')
+    string_data = serializers.serialize(
+        format,
+        [child_1.parent_ptr, child_2.parent_ptr, child_2, child_1],
+        use_natural_foreign_keys=True, use_natural_primary_keys=True,
+    )
+    child_1.delete()
+    child_2.delete()
+    for obj in serializers.deserialize(format, string_data):
+        obj.save()
+    children = Child.objects.all()
+    self.assertEqual(len(children), 2)
+    for child in children:
+        # If it's possible to find the superclass from the subclass and it's
+        # the correct superclass, it's working.
+        self.assertEqual(child.child_data, child.parent_data)
+
+
 # Dynamically register tests for each serializer
 register_tests(NaturalKeySerializerTests, 'test_%s_natural_key_serializer', natural_key_serializer_test)
 register_tests(NaturalKeySerializerTests, 'test_%s_serializer_natural_keys', natural_key_test)
+register_tests(NaturalKeySerializerTests, 'test_%s_serializer_natural_pks_mti', natural_pk_mti_test)