[Sqlalchemy-tickets] Issue #3137: Certain subqueries broken since 0.9.4 (zzzeek/sqlalchemy)

[thiefmaster <iss...@bi...>]

New issue 3137: Certain subqueries broken since 0.9.4
https://bitbucket.org/zzzeek/sqlalchemy/issue/3137/certain-subqueries-broken-since-094

thiefmaster:

The following query is causing an error inside the SQLAlchemy code since 0.9.4:

```
subquery = (select(['latitude'])
            .select_from(func.unnest(func.array_agg(Room.latitude))
                         .alias('latitude')).limit(1).as_scalar())
Room.query.with_entities(Room.building, subquery).group_by(Room.building).all()
```

Partial traceback:

```
  File ".../sqlalchemy/sql/selectable.py", line 429, in columns
    self._populate_column_collection()
  File ".../sqlalchemy/sql/selectable.py", line 992, in _populate_column_collection
    for col in self.element.columns._all_columns:
AttributeError: 'list' object has no attribute '_all_columns'
```

Related SO question with more details: http://stackoverflow.com/questions/24930155/strange-error-after-sqlalchemy-update-list-object-has-no-attribute-all-colu