Django auto_now Removal Gotcha

The latest Django revisions have removed the shortcut handlers "auto_now&quot and "auto_now_add". This didn’t look like a very big deal to replace. I simply did what was suggested in a django-users thread.

The problem was that after I did that, my live code seemed to work fine, but my test cases had several repeatable errors that I just couldn’t reproduce on the live server or at the command line.

The Fix

The fix involves overriding the "save" method, like so (quoting the thread)

class MyModel(models.Model):
    self.created = models.DatetimeField()
    self.modified = models.DatetimeField()

    def save(self):
        if not self.id:
            self.created = datetime.datetime.now()
        else:
            self.modified = datetime.datetime.now()

        super(MyModel, self).save()

What went wrong

After a lot of debugging and shrewd guessing, I finally had the eureka moment. The problem is that some of the test code was directly instantiating objects via calls like MyModel.objects.get_or_create(name="this", field="that"). In some places the test cases were using the even simpler MyModel.objects.create(name="this", field="that").

It finally occurred to me while walking the dog this morning that creating objects this way will skip the overridden "save()" method. I rushed to the computer to test my theory, and found that if I replaced:

MyModel.objects.get_or_create(name="this", field="that")

with this:

try:
    m = MyModel.objects.get(name="this", field="that")
except MyModel.DoesNotExist:
    m = MyModel(name="this", field="that")
    m.save()

everything worked perfectly.

So, watch out for that gotcha now that you can’t use auto_now in models any more.

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