`save()` and `sync()` having issues with objects fetched via a computed link

I used a computed link (which is actually a backlink) to fetch User and assotiated Group object. Then I used the actual forward `users` link on the Group object to remove the user.

See #927
* `test_modelgen_computed_01` - saving that modified group fails
* `test_modelgen_computed_02` - sync executes, but does not appear to actually make any changes in the database