[Sqlalchemy-tickets] Issue #4269: orm.Query() constructor raising "Boolean value of this clause is not defined" exception (zzzeek/sqlalchemy)

[Patrick R. <iss...@bi...>]

New issue 4269: orm.Query() constructor raising "Boolean value of this clause is not defined" exception
https://bitbucket.org/zzzeek/sqlalchemy/issues/4269/ormquery-constructor-raising-boolean-value

Patrick Rusk:

(This problem was noted in comments in https://github.com/zzzeek/sqlalchemy/commit/3fa38a1a2313b4644daa431d629394d6bb14497a but not followed up on.)

The `sqlalchemy.orm.Query()` class takes an `entities` parameter defined to be "a sequence of entities and/or SQL expressions", but it used to actual accept a single SQL expression as well. Now, some such expressions cause a "TypeError: Boolean value of this clause is not defined" exception to be raised.

The offending line is https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/orm/query.py#L150

As noted in the comments, changing it to `if entities is not None:` would probably fix the issue.

Test case:

```
#!python

from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer
import sqlalchemy.sql

class User(declarative_base()):
    __tablename__ = 'users'
    id = sqlalchemy.Column(sqlalchemy.Integer, primary_key=True)

select = sqlalchemy.sql.func.max(User.id).label('max')
query = sqlalchemy.orm.Query(select)
```
...works under 1.2.7, but generates this stack trace under 1.2.8...

```
#!python

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-1-cf50a8343d19> in <module>()
      8 
      9 select = sqlalchemy.sql.func.max(User.id).label('max')
---> 10 query = sqlalchemy.orm.Query(select)

/Users/patrick/.virtualenvs/shackleton/lib/python2.7/site-packages/sqlalchemy/orm/query.pyc in __init__(self, entities, session)
    140         self.session = session
    141         self._polymorphic_adapters = {}
--> 142         self._set_entities(entities)
    143 
    144     def _set_entities(self, entities, entity_wrapper=None):

/Users/patrick/.virtualenvs/shackleton/lib/python2.7/site-packages/sqlalchemy/orm/query.pyc in _set_entities(self, entities, entity_wrapper)
    148         self._primary_entity = None
    149         self._has_mapper_entities = False
--> 150         if entities:
    151             for ent in util.to_list(entities):
    152                 entity_wrapper(self, ent)

/Users/patrick/.virtualenvs/shackleton/lib/python2.7/site-packages/sqlalchemy/sql/elements.pyc in __bool__(self)
    485 
    486     def __bool__(self):
--> 487         raise TypeError("Boolean value of this clause is not defined")
    488 
    489     __nonzero__ = __bool__

TypeError: Boolean value of this clause is not defined
```
Anyone experiencing this can easily fix their issue by throwing [] around their expression, but it is a breaking change (of an undocumented feature).

(Please forgive the nonsensical test case above. I've never used SQLAlchemy. Just diagnosing a bug in someone else's code.)