SQLAlchemy with_for_update row locking not working?

Question

There is a student whose type attribute is 4 and the minimum value for type attribute can be 1.

In postgres

In se

Answer 1

The 2 sessions should look like this:

user = Student.query.with_for_update(of=Student, nowait=True).filter(Student.id == 122).first()
user.type = 1
db.session.commit()

and

user = Student.query.with_for_update(of=Student, nowait=True).filter(Student.id == 122).first()
user.type -= 1
db.session.commit()

In order for FOR UPDATE to work properly, all involved transactions which intend to update the row need to use it.

In your example, session 2 is not using with_for_update. Since you didn't tell it to use FOR UPDATE, it is free to read the old value of the row (since the new value has not yet been committed, and locks do not block pure readers), then modify it that in-memory value, then write it back.

If you don't want to use FOR UPDATE everywhere that you read row with the intention of changing it, you could instead use isolation level serializable everywhere. However if you do, things might not block, but rather will appear to succeed until the commit, then throw serialization errors that will need to be caught and dealt with.

Note: Your pre-edit example should have worked as both sessions were labelled with with_for_update.