Django transactions and concurrency

Question

I have a view that needs to perform updates to the database involving a shared resource that needs locking (the implementation is complex, but nothing more than a shared counter at heart).

To shield myself from race conditions, I'm using code that looks roughly like this:

@transaction.commit_manually
def do_it(request):
    affected_models = Something.objects.select_for_update(blah = 1)

    for model in affected_models:
        model.modify()
        model.save()

    transaction.commit()

Is this usage of commit_manually , select_for_update() and save() ok? How can I write a test that confirms this? I can't find, say, a signal that Django fires between transactions; and I can't just run it and hope concurrency issues arise and are dealt with.

Answer 1

Why not to use commit_on_success there?

I think the query itself should look like:

Something.objects.select_for_update().filter(...)

I don't think django does anything special on select_for_update what you could assert on. Only assertion comes to my head is assertTrue(queryset.query.select_for_update) . It tests nothing and might be usefull only if somebody accidentaly(?) removes the call.

Even if you come up with some unit test for this run condition problem I don't think it would be wise to put that into the project.

Rather focus on testing your code, not the db behaviour.