Django - Rollback save with transaction atomic

Question

I am trying to create a view where I save an object but I'd like to undo that save if some exception is raised. This is what I tried:

class MyView(View):

    @transation.atomic
    def post(self, request, *args, **kwargs):
        try:
            some_object = SomeModel(...)
            some_object.save()

            if something:
                raise exception.NotAcceptable()
                # When the workflow comes into this condition, I think the previous save should be undome
                # Whant am I missing?

        except exception.NotAcceptable, e:
            # do something

What am I doing wrong? even when the exception is raised some_object is still in DataBase.

Answer 1

Atomicity Documentation

To summarize, @transaction.atomic will execute a transaction on the database if your view produces a response without errors. Because you're catching the exception yourself, it appears to Django that your view executed just fine.

If you catch the exception, you need to handle it yourself: Controlling Transactions

If you need to produce a proper json response in the event of failure:

from django.db import SomeError, transaction

def viewfunc(request):
    do_something()

    try:
        with transaction.atomic():
            thing_that_might_fail()
    except SomeError:
        handle_exception()

    render_response()

Answer 2

However, if an exception happens in a function decorated with transaction.atomic, then you don't have anything to do, it'll rollback automatically to the savepoint created by the decorator before running the your function, as documented:

atomic allows us to create a block of code within which the atomicity on the database is guaranteed. If the block of code is successfully completed, the changes are committed to the database. If there is an exception, the changes are rolled back.

If the exception is catched in an except block, then it should be re-raised for atomic to catch it and do the rollback, ie.:

    try:
        some_object = SomeModel(...)
        some_object.save()

        if something:
            raise exception.NotAcceptable()
            # When the workflow comes into this condition, I think the previous save should be undome
            # Whant am I missing?

    except exception.NotAcceptable, e:
        # do something
        raise  # re-raise the exception to make transaction.atomic rollback

Also, if you want more control, you can rollback manually to previously set savepoint, ie.:

class MyView(View):
    def post(self, request, *args, **kwargs):
        sid = transaction.savepoint()
        some_object = SomeModel(...)
        some_object.save()

        if something:
            transaction.savepoint_rollback(sid)
        else:
            try:
                # In worst case scenario, this might fail too
                transaction.savepoint_commit(sid)
            except IntegrityError:
                transaction.savepoint_rollback(sid)