[Sqlalchemy-tickets] Issue #3956: Mutable + column properties broken (zzzeek/sqlalchemy)

[Marcin K. <iss...@bi...>]

New issue 3956: Mutable + column properties broken
https://bitbucket.org/zzzeek/sqlalchemy/issues/3956/mutable-column-properties-broken

Marcin Kurczewski:

Trying to use both mutable and column properties within the same table results in mapper exception. 

The stacktrace is:


```
#!text
Traceback (most recent call last):
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 682, in __getattr__
    return getattr(self.comparator, key)
AttributeError: 'Comparator' object has no attribute 'info'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/srv/www/tmp.sakuya.pl/f/test", line 25, in <module>
    session.query(User).all()
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/session.py", line 1328, in query
    return self._query_cls(entities, self, **kwargs)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 139, in __init__
    self._set_entities(entities)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 150, in _set_entities
    self._set_entity_selectables(self._entities)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 180, in _set_entity_selectables
    ent.setup_entity(*d[entity])
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/query.py", line 3595, in setup_entity
    self._with_polymorphic = ext_info.with_polymorphic_mappers
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/util/langhelpers.py", line 764, in __get__
    obj.__dict__[self.__name__] = result = self.fget(obj)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 1948, in _with_polymorphic_mappers
    configure_mappers()
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/mapper.py", line 2869, in configure_mappers
    mapper, mapper.class_)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/event/attr.py", line 218, in __call__
    fn(*args, **kw)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/orm/events.py", line 616, in wrap
    fn(*arg, **kw)
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/ext/mutable.py", line 603, in listen_for_type
    prop.columns[0].info.get('_ext_mutable_orig_type')
  File "/home/rr-/.local/lib/python3.6/site-packages/sqlalchemy/sql/elements.py", line 688, in __getattr__
    key)
AttributeError: Neither 'Label' object nor 'Comparator' object has an attribute 'info'
```


The code works fine on 1.1.7 and doesn't on 1.1.8. The GitHub hash of the change that introduces this behavior is 07b63894cb8ff9529b406f196b5d7cb9af209e9e: "Track SchemaEventTarget types in as_mutable()".

I attach a code sample that triggers this behavior.