[j-kidd]

I am pretty sure SQLAlchemy will translate that into:

    UPDATE purchase SET downloads_left=%(downloads_left)s WHERE purchase.id = %(purchases_id)s;

By the way, the posted code has an off-by-one error, as it should do the checking first before the minus operation. Also, the line `db.session.add(purchase)` is redundant.

EDIT: remove bad sample