Refs #31051 -- Fixed reloading the database with circular related objects and natural keys for tests.

Made deserialize_db_from_string() do not sort dependencies.

deserialize_db_from_string() doesn't use natural keys, so there is no
need to sort dependencies in serialize_db_to_string(). Moreover,
sorting models cause issues for circular dependencies.

3 files changed, 46 insertions, 5 deletions

diff --git a/django/db/backends/base/creation.py b/django/db/backends/base/creation.py
index 230954ddfc..a19ec48419 100644
--- a/django/db/backends/base/creation.py
+++ b/django/db/backends/base/creation.py
@@ -97,21 +97,21 @@ class BaseDatabaseCreation:
         Designed only for test runner usage; will not handle large
         amounts of data.
         """
-        # Build list of all apps to serialize
+        # Build list of all models to serialize.
         from django.db.migrations.loader import MigrationLoader
         loader = MigrationLoader(self.connection)
-        app_list = []
+        model_list = []
         for app_config in apps.get_app_configs():
             if (
                 app_config.models_module is not None and
                 app_config.label in loader.migrated_apps and
                 app_config.name not in settings.TEST_NON_SERIALIZED_APPS
             ):
-                app_list.append((app_config, None))
+                model_list.extend(app_config.get_models())
 
         # Make a function to iteratively return every object
         def get_objects():
-            for model in serializers.sort_dependencies(app_list):
+            for model in model_list:
                 if (model._meta.can_migrate(self.connection) and
                         router.allow_migrate_model(self.connection.alias, model)):
                     queryset = model._default_manager.using(self.connection.alias).order_by(model._meta.pk.name)
diff --git a/tests/backends/base/test_creation.py b/tests/backends/base/test_creation.py
index 19a3915021..01215a9a5b 100644
--- a/tests/backends/base/test_creation.py
+++ b/tests/backends/base/test_creation.py
@@ -7,7 +7,9 @@ from django.db.backends.base.creation import (
 )
 from django.test import SimpleTestCase, TransactionTestCase
 
-from ..models import Object, ObjectReference, ObjectSelfReference
+from ..models import (
+    CircularA, CircularB, Object, ObjectReference, ObjectSelfReference,
+)
 
 
 def get_connection_copy():
@@ -123,3 +125,26 @@ class TestDeserializeDbFromString(TransactionTestCase):
         obj_2 = ObjectSelfReference.objects.get(key='Y')
         self.assertEqual(obj_1.obj, obj_2)
         self.assertEqual(obj_2.obj, obj_1)
+
+    def test_circular_reference_with_natural_key(self):
+        # serialize_db_to_string() and deserialize_db_from_string() handles
+        # circular references for models with natural keys.
+        obj_a = CircularA.objects.create(key='A')
+        obj_b = CircularB.objects.create(key='B', obj=obj_a)
+        obj_a.obj = obj_b
+        obj_a.save()
+        # Serialize objects.
+        with mock.patch('django.db.migrations.loader.MigrationLoader') as loader:
+            # serialize_db_to_string() serializes only migrated apps, so mark
+            # the backends app as migrated.
+            loader_instance = loader.return_value
+            loader_instance.migrated_apps = {'backends'}
+            data = connection.creation.serialize_db_to_string()
+        CircularA.objects.all().delete()
+        CircularB.objects.all().delete()
+        # Deserialize objects.
+        connection.creation.deserialize_db_from_string(data)
+        obj_a = CircularA.objects.get()
+        obj_b = CircularB.objects.get()
+        self.assertEqual(obj_a.obj, obj_b)
+        self.assertEqual(obj_b.obj, obj_a)
diff --git a/tests/backends/models.py b/tests/backends/models.py
index 87809b84a8..096fdb57cc 100644
--- a/tests/backends/models.py
+++ b/tests/backends/models.py
@@ -107,6 +107,22 @@ class ObjectSelfReference(models.Model):
     obj = models.ForeignKey('ObjectSelfReference', models.SET_NULL, null=True)
 
 
+class CircularA(models.Model):
+    key = models.CharField(max_length=3, unique=True)
+    obj = models.ForeignKey('CircularB', models.SET_NULL, null=True)
+
+    def natural_key(self):
+        return (self.key,)
+
+
+class CircularB(models.Model):
+    key = models.CharField(max_length=3, unique=True)
+    obj = models.ForeignKey('CircularA', models.SET_NULL, null=True)
+
+    def natural_key(self):
+        return (self.key,)
+
+
 class RawData(models.Model):
     raw_data = models.BinaryField()