Clarify .build() issue with Django>1.8 (Ref #198).

From 1.8 onwards, this crashes:

>>> a = MyModel()  # Don't save
>>> b = MyOtherModel(fkey_to_mymodel=a)

In turn, it breaks:

    class MyModelFactory(factory.django.DjangoModelFactory):
        class Meta:
            model = MyModel

    class MyOtherModelFactory(factory.django.DjangoModelFactory):
        class Meta:
            model = MyOtherModel
        fkey_to_mymodel = factory.SubFactory(MyModelFactory)

    MyOtherModelFactory.build()  # Breaks

The error message is: Cannot assign "MyModel()": "MyModel" instance isn't saved in the database.

See https://code.djangoproject.com/ticket/10811 for details.

docs/orms.rst

@@ -40,6 +40,14 @@ All factories for a Django :class:`~django.db.models.Model` should use the
       once all post-generation hooks have run.
 
 
+.. note:: Starting with Django 1.8, it is no longer possible to call ``.build()``
+          on a factory if this factory uses a :class:`~factory.SubFactory` pointing
+          to another model: Django refuses to set a :class:`~djang.db.models.ForeignKey`
+          to an unsaved :class:`~django.db.models.Model` instance.
+
+          See https://code.djangoproject.com/ticket/10811 for details.
+
+
 .. class:: DjangoOptions(factory.base.FactoryOptions)
 
     The ``class Meta`` on a :class:`~DjangoModelFactory` supports extra parameters:

tests/djapp/models.py

@@ -68,6 +68,15 @@ class StandardSon(StandardModel):
     pass
 
 
+class PointedModel(models.Model):
+    foo = models.CharField(max_length=20)
+
+
+class PointingModel(models.Model):
+    foo = models.CharField(max_length=20)
+    pointed = models.OneToOneField(PointedModel, related_name='pointer', null=True)
+
+
 WITHFILE_UPLOAD_TO = 'django'
 WITHFILE_UPLOAD_DIR = os.path.join(settings.MEDIA_ROOT, WITHFILE_UPLOAD_TO)
 

tests/test_django.py

@@ -361,6 +361,32 @@ def test_auto_sequence(self):
         self.assertEqual(1, obj.pk)
 
 
+@unittest.skipIf(django is None, "Django not installed.")
+class DjangoRelatedFieldTestCase(django_test.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        super(DjangoRelatedFieldTestCase, cls).setUpClass()
+        class PointedFactory(factory.django.DjangoModelFactory):
+            class Meta:
+                model = models.PointedModel
+            foo = 'ahah'
+
+        class PointerFactory(factory.django.DjangoModelFactory):
+            class Meta:
+                model = models.PointingModel
+            pointed = factory.SubFactory(PointedFactory, foo='hihi')
+            foo = 'bar'
+
+        cls.PointedFactory = PointedFactory
+        cls.PointerFactory = PointerFactory
+
+    def test_direct_related_create(self):
+        ptr = self.PointerFactory()
+        self.assertEqual('hihi', ptr.pointed.foo)
+        self.assertEqual(ptr.pointed, models.PointedModel.objects.get())
+
+
 @unittest.skipIf(django is None, "Django not installed.")
 class DjangoFileFieldTestCase(unittest.TestCase):