Removed some usage of _sa_class_manager

1) Removed a usage of `_sa_class_manager` since it didn't represent the full model. 
2) When relation doesn't exists in the `_sa_class_manager`, find it elsewhere in the instance.
3) Don't add a hybrid property which is a relation to the `columns`

 Fixes issue #319
No, there's some other code that depends on the return value from _sa_class_manager