Django - AutoField with regards to a foreign key

Question

I have a model with a unique integer that needs to increment with regards to a foreign key, and the following code is how I currently handle it:

class MyModel(models.Model):
    business = models.ForeignKey(Business)
    number = models.PositiveIntegerField()
    spam = models.CharField(max_length=255)

    class Meta:
        unique_together = (('number', 'business'),)

    def save(self, *args, **kwargs):
        if self.pk is None:  # New instance's only
            try:
                highest_number = MyModel.objects.filter(business=self.business).order_by('-number').all()[0].number
                self.number = highest_number + 1
            except ObjectDoesNotExist:  # First MyModel instance
                self.number = 1
        super(MyModel, self).save(*args, **kwargs)

I have the following questions regarding this:

1. Multiple people can create MyModel instances for the same business, all over the internet. Is it possible for 2 people creating MyModel instances at the same time, and .count() returns 500 at the same time for both, and then both try to essentially set self.number = 501 at the same time (raising an IntegrityError)? The answer seems like an obvious "yes, it could happen", but I had to ask.
2. Is there a shortcut, or "Best way" to do this, which I can use (or perhaps a SuperAutoField that handles this)?

I can't just slap a while model_not_saved: try:, except IntegrityError: in, because other restraints in the model could lead to an endless loop, and a disaster worse than Chernobyl (maybe not quite that bad).

Answer 1

You want that constraint at the database level. Otherwise you're going to eventually run into the concurrency problem you discussed. The solution is to wrap the entire operation (read, increment, write) in a transaction.

Why can't you use an AutoField for instead of a PositiveIntegerField?

number = models.AutoField()

However, in this case number is almost certainly going to equal yourmodel.id, so why not just use that?

Edit:

Oh, I see what you want. You want a numberfield that doesn't increment unless there's more than one instance of MyModel.business.

I would still recommend just using the id field if you can, since it's certain to be unique. If you absolutely don't want to do that (maybe you're showing this number to users), then you will need to wrap your save method in a transaction.

You can read more about transactions in the docs:

http://docs.djangoproject.com/en/dev/topics/db/transactions/

If you're just using this to count how many instances of MyModel have a FK to Business, you should do that as a query rather than trying to store a count.