How to update SQLAlchemy row entry?

There are several ways to UPDATE using sqlalchemy

1) user.no_of_logins += 1   session.commit()2) session.query().\       filter(User.username == form.username.data).\       update({"no_of_logins": (User.no_of_logins +1)})   session.commit()3) conn = engine.connect()   stmt = User.update().\       values(no_of_logins=(User.no_of_logins + 1)).\       where(User.username == form.username.data)   conn.execute(stmt)4) setattr(user, 'no_of_logins', user.no_of_logins+1)   session.commit()

user.no_of_logins += 1session.commit()

Examples to clarify the important issue in accepted answer's comments

I didn't understand it until I played around with it myself, so I figured there would be others who were confused as well. Say you are working on the user whose id == 6 and whose no_of_logins == 30 when you start.

# 1 (bad)user.no_of_logins += 1# result: UPDATE user SET no_of_logins = 31 WHERE user.id = 6# 2 (bad)user.no_of_logins = user.no_of_logins + 1# result: UPDATE user SET no_of_logins = 31 WHERE user.id = 6# 3 (bad)setattr(user, 'no_of_logins', user.no_of_logins + 1)# result: UPDATE user SET no_of_logins = 31 WHERE user.id = 6# 4 (ok)user.no_of_logins = User.no_of_logins + 1# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6# 5 (ok)setattr(user, 'no_of_logins', User.no_of_logins + 1)# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6

The point

By referencing the class instead of the instance, you can get SQLAlchemy to be smarter about incrementing, getting it to happen on the database side instead of the Python side. Doing it within the database is better since it's less vulnerable to data corruption (e.g. two clients attempt to increment at the same time with a net result of only one increment instead of two). I assume it's possible to do the incrementing in Python if you set locks or bump up the isolation level, but why bother if you don't have to?

A caveat

If you are going to increment twice via code that produces SQL like SET no_of_logins = no_of_logins + 1, then you will need to commit or at least flush in between increments, or else you will only get one increment in total:

# 6 (bad)user.no_of_logins = User.no_of_logins + 1user.no_of_logins = User.no_of_logins + 1session.commit()# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6# 7 (ok)user.no_of_logins = User.no_of_logins + 1session.flush()# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6user.no_of_logins = User.no_of_logins + 1session.commit()# result: UPDATE user SET no_of_logins = no_of_logins + 1 WHERE user.id = 6