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.

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()