Fix Python – Django auto_now and auto_now_add

Question

Asked By – Paul Tarjan

For Django 1.1.

I have this in my models.py:

class User(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    modified = models.DateTimeField(auto_now=True)

When updating a row I get:

[Sun Nov 15 02:18:12 2009] [error] /home/ptarjan/projects/twitter-meme/django/db/backends/mysql/base.py:84: Warning: Column 'created' cannot be null
[Sun Nov 15 02:18:12 2009] [error]   return self.cursor.execute(query, args)

The relevant part of my database is:

  `created` datetime NOT NULL,
  `modified` datetime NOT NULL,

Is this cause for concern?

Side question: in my admin tool, those two fields aren’t showing up. Is that expected?

Now we will see solution for issue: Django auto_now and auto_now_add


Answer

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.

This question is answered By – jathanism

This answer is collected from stackoverflow and reviewed by FixPython community admins, is licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0