[1.5.x] Fixed #19688 -- Allow model subclassing with a custom metaclass using six.with_metaclass

Backport of 6b03179e126d4df01623dccc162c1579f349e41e from master.

Although we're post RC 2, I'm backporting this because it's arguably a
major bug in a new feauture that will prevent several well-known
third-party apps from being ported to Python 3.

4 files changed, 52 insertions, 2 deletions

diff --git a/django/db/models/base.py b/django/db/models/base.py
index 38afc60991..a58761b643 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -58,12 +58,21 @@ class ModelBase(type):
     """
     def __new__(cls, name, bases, attrs):
         super_new = super(ModelBase, cls).__new__
+
         # six.with_metaclass() inserts an extra class called 'NewBase' in the
-        # inheritance tree: Model -> NewBase -> object. Ignore this class.
+        # inheritance tree: Model -> NewBase -> object. But the initialization
+        # should be executed only once for a given model class.
+
+        # attrs will never be empty for classes declared in the standard way
+        # (ie. with the `class` keyword). This is quite robust.
+        if name == 'NewBase' and attrs == {}:
+            return super_new(cls, name, bases, attrs)
+
+        # Also ensure initialization is only performed for subclasses of Model
+        # (excluding Model class itself).
         parents = [b for b in bases if isinstance(b, ModelBase) and
                 not (b.__name__ == 'NewBase' and b.__mro__ == (b, object))]
         if not parents:
-            # If this isn't a subclass of Model, don't do anything special.
             return super_new(cls, name, bases, attrs)
 
         # Create the class.
diff --git a/tests/modeltests/base/__init__.py b/tests/modeltests/base/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/tests/modeltests/base/__init__.py
diff --git a/tests/modeltests/base/models.py b/tests/modeltests/base/models.py
new file mode 100644
index 0000000000..3d39302aba
--- /dev/null
+++ b/tests/modeltests/base/models.py
@@ -0,0 +1,5 @@
+from django.db.models.base import ModelBase
+
+
+class CustomBaseModel(ModelBase):
+    pass
diff --git a/tests/modeltests/base/tests.py b/tests/modeltests/base/tests.py
new file mode 100644
index 0000000000..6229b7a305
--- /dev/null
+++ b/tests/modeltests/base/tests.py
@@ -0,0 +1,36 @@
+from __future__ import unicode_literals
+
+from django.db import models
+from django.test.testcases import SimpleTestCase
+from django.utils import six
+from django.utils.unittest import skipIf
+
+from .models import CustomBaseModel
+
+
+class CustomBaseTest(SimpleTestCase):
+
+    @skipIf(six.PY3, 'test metaclass definition under Python 2')
+    def test_py2_custom_base(self):
+        """
+        Make sure models.Model can be subclassed with a valid custom base
+        using __metaclass__
+        """
+        try:
+            class MyModel(models.Model):
+                __metaclass__ = CustomBaseModel
+        except Exception:
+            self.fail("models.Model couldn't be subclassed with a valid "
+                      "custom base using __metaclass__.")
+
+    def test_six_custom_base(self):
+        """
+        Make sure models.Model can be subclassed with a valid custom base
+        using `six.with_metaclass`.
+        """
+        try:
+            class MyModel(six.with_metaclass(CustomBaseModel, models.Model)):
+                pass
+        except Exception:
+            self.fail("models.Model couldn't be subclassed with a valid "
+                      "custom base using `six.with_metaclass`.")