Fixed #26552 -- Deferred constraint checks when reloading the database with data for tests.

deserialize_db_from_string() loads the full serialized database
contents, which might contain forward references and cycles. That
caused IntegrityError because constraints were checked immediately.

Now, it loads data in a transaction with constraint checks deferred
until the end of the transaction.

django/db/backends/base/creation.py

@@ -6,6 +6,7 @@ from django.apps import apps
 from django.conf import settings
 from django.core import serializers
 from django.db import router
+from django.db.transaction import atomic
 
 # The prefix to put on the default database name when creating
 # the test database.
@@ -126,8 +127,16 @@ class BaseDatabaseCreation:
         the serialize_db_to_string() method.
         """
         data = StringIO(data)
-        for obj in serializers.deserialize("json", data, using=self.connection.alias):
-            obj.save()
+        # Load data in a transaction to handle forward references and cycles.
+        with atomic(using=self.connection.alias):
+            # Disable constraint checks, because some databases (MySQL) doesn't
+            # support deferred checks.
+            with self.connection.constraint_checks_disabled():
+                for obj in serializers.deserialize('json', data, using=self.connection.alias):
+                    obj.save()
+            # Manually check for any invalid keys that might have been added,
+            # because constraint checks were disabled.
+            self.connection.check_constraints()
 
     def _get_database_display_str(self, verbosity, database_name):
         """

tests/backends/base/test_creation.py

@@ -7,6 +7,8 @@ from django.db.backends.base.creation import (
 )
 from django.test import SimpleTestCase
 
+from ..models import Object, ObjectReference
+
 
 def get_connection_copy():
     # Get a copy of the default connection. (Can't use django.db.connection
@@ -73,3 +75,29 @@ class TestDbCreationTests(SimpleTestCase):
         finally:
             with mock.patch.object(creation, '_destroy_test_db'):
                 creation.destroy_test_db(old_database_name, verbosity=0)
+
+
+class TestDeserializeDbFromString(SimpleTestCase):
+    databases = {'default'}
+
+    def test_circular_reference(self):
+        # deserialize_db_from_string() handles circular references.
+        data = """
+        [
+            {
+                "model": "backends.object",
+                "pk": 1,
+                "fields": {"obj_ref": 1, "related_objects": []}
+            },
+            {
+                "model": "backends.objectreference",
+                "pk": 1,
+                "fields": {"obj": 1}
+            }
+        ]
+        """
+        connection.creation.deserialize_db_from_string(data)
+        obj = Object.objects.get()
+        obj_ref = ObjectReference.objects.get()
+        self.assertEqual(obj.obj_ref, obj_ref)
+        self.assertEqual(obj_ref.obj, obj)

tests/backends/models.py

@@ -89,6 +89,7 @@ class Item(models.Model):
 
 class Object(models.Model):
     related_objects = models.ManyToManyField("self", db_constraint=False, symmetrical=False)
+    obj_ref = models.ForeignKey('ObjectReference', models.CASCADE, null=True)
 
     def __str__(self):
         return str(self.id)