[Sqlalchemy-tickets] Issue #4069: session.merge() can miss object gc'ed after checking in identity map (zzzeek/sqlalchemy)

[Michael B. <iss...@bi...>]

New issue 4069: session.merge() can miss object gc'ed after checking in identity map
https://bitbucket.org/zzzeek/sqlalchemy/issues/4069/sessionmerge-can-miss-object-gced-after

Michael Bayer:

sample trace:

```
#!


   user = session.merge(cachedUser, load=False)
 File "/venv/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 157, in do
   return getattr(self.registry(), name)(*args, **kwargs)
 File "/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1849, in merge
   _resolve_conflict_map=_resolve_conflict_map)
 File "/venv/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 1875, in _merge
   merged = self.identity_map[key]
 File "/venv/lib/python2.7/site-packages/sqlalchemy/orm/identity.py", line 89, in __getitem__
   state = self._dict[key]
KeyError: <snip>
```

moved from #4030