Difference between auto_now and auto_now_add

问题

What I understood in Django models field's attributes is

auto_now - updates the value of field to current time and date every time the Model.save() is called.
auto_now_add - updates the value with the time and date of creation of record.

My question is what if a filed in model contains both the auto_now and auto_now_add set to True? What happens in that case?

回答1:

auto_now takes precedence (obviously, because it updates field each time, while auto_now_add updates on creation only). Here is the code for DateField.pre_save method:

def pre_save(self, model_instance, add):
    if self.auto_now or (self.auto_now_add and add):
        value = datetime.date.today()
        setattr(model_instance, self.attname, value)
        return value
    else:
        return super().pre_save(model_instance, add)

As you can see, if auto_now is set or both auto_now_add is set and the object is new, the field will receive current day.

The same for DateTimeField.pre_save:

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

回答2:

According to the django documentation using but auto_now and auto_now_add as True will result in an error because they are both mutually exclusive.