[1.7.x] Fixed #23418 -- Fail when migration deconstruct produces invalid import

Backport of d28b5f13b3 from master

django/utils/deconstruct.py

@@ -1,3 +1,5 @@
+from importlib import import_module
+
 def deconstructible(*args, **kwargs):
     """
     Class decorator that allow the decorated class to be serialized
@@ -19,8 +21,25 @@ def deconstructible(*args, **kwargs):
             Returns a 3-tuple of class import path, positional arguments,
             and keyword arguments.
             """
+            # Python 2/fallback version
+            if path:
+                module_name, _, name = path.rpartition('.')
+            else:
+                module_name = obj.__module__
+                name = obj.__class__.__name__
+            # Make sure it's actually there and not an inner class
+            module = import_module(module_name)
+            if not hasattr(module, name):
+                raise ValueError(
+                    "Could not find object %s in %s.\n"
+                    "Please note that you cannot serialize things like inner "
+                    "classes. Please move the object into the main module "
+                    "body to use migrations.\n"
+                    "For more information, see "
+                    "https://docs.djangoproject.com/en/dev/topics/migrations/#serializing-values"
+                    % (name, module_name))
             return (
-                path or '%s.%s' % (obj.__class__.__module__, obj.__class__.__name__),
+                path or '%s.%s' % (obj.__class__.__module__, name),
                 obj._constructor_args[0],
                 obj._constructor_args[1],
             )

docs/releases/1.7.1.txt

@@ -20,3 +20,6 @@ Bugfixes
 * Fixed serialization of ``type`` objects in migrations (:ticket:`22951`).
 
 * Allowed inline and hidden references to admin fields (:ticket:`23431`).
+
+* The ``@deconstructible`` decorator now fails with a ``ValueError`` if the
+  decorated object cannot automatically be imported (:ticket:`23418`).

tests/migrations/test_writer.py

@@ -167,9 +167,17 @@ class WriterTests(TestCase):
         self.assertEqual(string, "django.core.validators.EmailValidator(message='hello')")
         self.serialize_round_trip(validator)
 
-        validator = deconstructible(path="custom.EmailValidator")(EmailValidator)(message="hello")
+        validator = deconstructible(path="migrations.test_writer.EmailValidator")(EmailValidator)(message="hello")
         string = MigrationWriter.serialize(validator)[0]
-        self.assertEqual(string, "custom.EmailValidator(message='hello')")
+        self.assertEqual(string, "migrations.test_writer.EmailValidator(message='hello')")
+
+        validator = deconstructible(path="custom.EmailValidator")(EmailValidator)(message="hello")
+        with self.assertRaisesMessage(ImportError, "No module named 'custom'"):
+            MigrationWriter.serialize(validator)
+
+        validator = deconstructible(path="django.core.validators.EmailValidator2")(EmailValidator)(message="hello")
+        with self.assertRaisesMessage(ValueError, "Could not find object EmailValidator2 in django.core.validators."):
+            MigrationWriter.serialize(validator)
 
     def test_serialize_empty_nonempty_tuple(self):
         """