Question

Assume I have a Django model with the following fields:

class Gizmo(models.Model):
    date_created = models.DateTimeField(auto_now_add=True)
    date_updated = models.DateTimeField(auto_now=True)

When I create a gizmo, can I be sure that Django will generate identical timestamps for the two fields? I’m worried that Django might create a datetime instance for each, and that these will produce slightly different timestamps. It’s important that the timestamps be identical so that there’s a reliable way to determine whether the gizmo has been updated (i.e. it has been if the timestamps differ).

Answer 1

Based on one test I can tell you the answer is no.

test = TestModel.objects.create()
print test.date_created
# Out: datetime.datetime(2011, 12, 12, 20, 51, 41, 620985)
print test.date_last_modified
# Out: datetime.datetime(2011, 12, 12, 20, 51, 41, 621008)

I’d just override the save method on your model and populate these values myself.

PS: I did just look at the field code first, but ended up just running the actual test since I wouldn’t have known if django does something on a model level somewhere if there are two auto_now-type fields or something.

# datetimefield
def pre_save(self, model_instance, add):
    if self.auto_now or (self.auto_now_add and add):
        value = datetime.datetime.now()
        setattr(model_instance, self.attname, value)
        return value
    else:
        return super(DateTimeField, self).pre_save(model_instance, add)