Add support for .count() to Baked Queries

[Marcus Cobden]

The Baked Query API has support for all, first, get & one, to match the Query API, but no count method.

I could really use a count method right now! ;)

[Mike Bayer]

here's how to do that:

    bq = self.bakery(lambda s: s.query(User))

    sess = Session()

    def count(q):
        col = func.count(literal_column('*'))
        return q.from_self(col)

    bq += count

    result = bq(session).first()[0]

[Mike Bayer]

https://gerrit.sqlalchemy.org/299

[Marcus Cobden]

That's brilliant, thanks!

[Mike Bayer]

Add count(), scalar() to baked query

Change-Id: I8af0d7b41ae2df384ce5d0ef274732352d81f376 Fixes: #3897

→ <<cset ecfda6512cf8>>