[Sqlalchemy-tickets] Issue #3017: relationship loading strategy `expired` (zzzeek/sqlalchemy)

[Ronny P. <iss...@bi...>]

New issue 3017: relationship loading strategy `expired`
https://bitbucket.org/zzzeek/sqlalchemy/issue/3017/relationship-loading-strategy-expired

Ronny Pfannschmidt:

currently all relationship loading loads the object on access

to use such objects without hitting the database,
i propose a loading strategy which returns an expired instance
with only the identity key set

roguhly the equivalent of

```
def hull(session, class_, *pk):
  mapper = class_mapper(class_)
  obj = class_()
  inspect(obj).key = mapper.identity_key_from_primary_key(pk)
  session.add(obj)
  session.expire(obj)
  return obj
```