Improves SQLAlchemy has_key method using SQL EXISTS clause

[jvrsantacruz]

It uses SQL EXISTS clause instead of fetching a record and comparing it against None.

See the current query:

 return None != self.bind.execute(
        select([self.table.c.key], self.table.c.key == key).limit(1)
    ).first()

This fragment of code can result a bit problematic due to de comparision
of a sqlalchemy RowProxy against None, which should be using the is operator also.

The proposed query:

return self.bind.execute(
    select([exists().where(self.table.c.key == key)])
).scalar()

Returns a boolean directly from the database,
and it should be allegedly faster while keeping it ANSI SQL.

Congrats for the library, it's easy to use and fun to extend.

[mbr]

Good catch. I'll merge this - have you run into any issues with this or can this wait until the next release?

[jvrsantacruz]

Well, now that you mention it, I looked into this because of this exception:

ipdb> None != self.bind.execute(
   select([self.table.c.key], self.table.c.key == key).limit(1)).first()
*** TypeError: 'NoneType' object is not iterable

Where the value returned by execute is a sqlalchemy RowProxy:

ipdb> value = self.bind.execute(
  select([self.table.c.key], self.table.c.key == key).limit(1)).first()
ipdb> value
(u'httphost',)
ipdb> type(value)
<class 'sqlalchemy.engine.result.RowProxy'>
ipdb> None != value
*** TypeError: 'NoneType' object is not iterable

This happens using SQLAlchemy (0.9.0).
After I upgraded to SQLAlchemy (0.9.7), problem was solved, and it behaves as expected.

So no, there is no hurry to get this out in a new version, I can work perfectly well.
Were someone to run into this very same error, the only thing it has to do is upgrade versions.

Anyways, the None against RowProxy comparision didn't felt like a very trustable behaviour to me, so I thougth that getting a good old bool was safer (and a little bit faster).

Thanks!