How to do to a manual commit using Django

Question

I would like to execute a transaction to delete some values, after that, count in db and if the result is < 1, rollback, i tried the following code:

@login_required
@csrf_exempt
@transaction.atomic
def update_user_groups(request):
    if request.POST:
        userId = request.POST['userId']
        groups = request.POST.getlist('groups[]')
        result = None
        with transaction.atomic():
            try:
                GroupsUsers.objects.filter(user_id=int(userId)).delete()
                for group in groups:
                    group_user = GroupsUsers()
                    group_user.user_id = userId
                    group_user.group_id = group
                    group_user.save()
                count = UsersInAdmin.objects.all().count()
                if count < 1:
                    transaction.commit()
                else:
                    transaction.rollback()
            except Exception, e:
                result = e
    return JsonResponse(result, safe=False)

Thanks,

Do you mean a data migration?.. – Sayse Apr 15 '16 at 18:10 — Sayse, Apr 15 '16 at 18:10

score 5 · Accepted Answer · answered Apr 15 '16 at 19:04

It's not possible to manually commit or rollback the transaction inside an atomic block.

Instead, you can raise an exception inside the atomic block. If the block completes, the transaction will be committed. If you raise an exception, the transaction will be rolled back. Outside the atomic block, you can catch the exception and carry on your view.

try:
    with transaction.atomic():
        do_stuff()
        if ok_to_commit():
            pass
        else:
            raise ValueError()
except ValueError:
    pass

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

I think you should give the Django documentation about db transactions another look.

The most relevant remark is the following:

Avoid catching exceptions inside atomic!

When exiting an atomic block, Django looks at whether it’s exited normally or with an exception to determine whether to commit or roll back. If you catch and handle exceptions inside an atomic block, you may hide from Django the fact that a problem has happened. This can result in unexpected behavior.

Adapting the example from the docs, it looks like you should nest your try and with blocks like this:

@transaction.atomic
def update_user_groups(request):
    # set your variables

    try:
        with transaction.atomic():
            # do your for loop
            if count < 1:
                # You should probably make a more descriptive, non-generic exception
                # see http://stackoverflow.com/questions/1319615/proper-way-to-declare-custom-exceptions-in-modern-python for more info
                raise Exception('Count is less than one')
    except:
        handle_exception()

How to do to a manual commit using Django

2 Answers2

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