Fix nullify generic relationship

sqlalchemy_utils/generic.py

@@ -50,6 +50,11 @@ class GenericAttributeImpl(attributes.ScalarAttributeImpl):
         # Set us on the state.
         dict_[self.key] = initiator
 
+        if initiator is None:
+            # Nullify relationship args
+            dict_[self.parent_token.id.key] = None
+            dict_[self.parent_token.discriminator.key] = None
+        else:
             # Get the primary key of the initiator and ensure we
             # can support this assignment.
             mapper = class_mapper(type(initiator))

tests/test_generic.py

@@ -28,6 +28,10 @@ class TestGenericForiegnKey(TestCase):
         self.User = User
         self.Event = Event
 
+    def test_set_as_none(self):
+        event = self.Event()
+        event.object = None
+
     def test_set_manual_and_get(self):
         user = self.User()