How to increase a counter in SQLAlchemy

Question

Suppose I have table tags which has a field count that indicates how many items have been tagged with the given tag.

How do I

Answer 1

If you are using the SQL layer, then you can use arbitrary SQL expressions in the update statement:

conn.execute(tags.update(tags.c.tag_id == 5).values(count=tags.c.count + 1))

The ORM Query object also has an update method:

session.query(Tag).filter_by(tag_id=5).update({'count': Tag.count + 1})

The ORM version is smart enough to also update the count attribute on the object itself if it's in the session.