Django Auto_Now and Auto_Now_Add

Django auto_now and auto_now_add

Any field with the auto_now attribute set will also inherit editable=False and therefore will not show up in the admin panel. There has been talk in the past about making the auto_now and auto_now_add arguments go away, and although they still exist, I feel you're better off just using a custom save() method.

So, to make this work properly, I would recommend not using auto_now or auto_now_add and instead define your own save() method to make sure that created is only updated if id is not set (such as when the item is first created), and have it update modified every time the item is saved.

I have done the exact same thing with other projects I have written using Django, and so your save() would look like this:

from django.utils import timezone

class User(models.Model):
    created     = models.DateTimeField(editable=False)
    modified    = models.DateTimeField()

    def save(self, *args, **kwargs):
        ''' On save, update timestamps '''
        if not self.id:
            self.created = timezone.now()
        self.modified = timezone.now()
        return super(User, self).save(*args, **kwargs)

Hope this helps!

Edit in response to comments:

The reason why I just stick with overloading save() vs. relying on these field arguments is two-fold:

1. The aforementioned ups and downs with their reliability. These arguments are heavily reliant on the way each type of database that Django knows how to interact with treats a date/time stamp field, and seems to break and/or change between every release. (Which I believe is the impetus behind the call to have them removed altogether).
2. The fact that they only work on DateField, DateTimeField, and TimeField, and by using this technique you are able to automatically populate any field type every time an item is saved.
3. Use django.utils.timezone.now() vs. datetime.datetime.now(), because it will return a TZ-aware or naive datetime.datetime object depending on settings.USE_TZ.

To address why the OP saw the error, I don't know exactly, but it looks like created isn't even being populated at all, despite having auto_now_add=True. To me it stands out as a bug, and underscores item #1 in my little list above: auto_now and auto_now_add are flaky at best.

Difference between auto_now and auto_now_add

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)

Django Models auto_now saving same time as auto_now_add

TL;DR: when you create an object for the first time, both created_at and updated_at will be filled with values, accordingly to the code you crafted.

• The auto_now_add will set the timezone.now() only when the instance is created.
• The auto_now will update the field every time the save method is called.

However, both functions the same way by triggering the field update event with timezone.now().

This means that when you create an object for the first time, both created_at and updated_at will be filled with values.

But, the next time you save that object, the created_at will remain the same, while the updated_at will call the timezone.now().

The options auto_now_add, auto_now, and default are mutually exclusive. Any combination of these options will result in an error.

Django docs: https://docs.djangoproject.com/en/4.0/ref/models/fields/

Last Update field in Django... 'auto_now' or 'hidden' widget

auto_now Automatically set the field to now every time the object is saved.

So to save updated record time auto_now is the best option

date_created = models.DateTimeField(auto_now_add=True)
date_updated = models.DateTimeField(auto_now=True)

https://docs.djangoproject.com/en/3.1/ref/models/fields/#datefield

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