Best way to add common date_added, date_modified to many models in Django

Question

I am adding date_added and date_modified fields to a bunch of common models in my current project. I am subclassing models.Model and adding the appropriate fields, but I want to add automated save behavior (i.e: evey time anyone calls MyModel.save(), the date_modified field gets updated. I see two approaches: overriding the save() method or adding a pre_save signal handler in the abstract base class.

class CommonData(models.Model):
    date_added = models.DateTimeField(default=datetime.datetime.today,null=False,blank=False)
    date_modified = models.DateTimeField(default=datetime.datetime.today,null=True,blank=True)

    # register a handler for the pre_save to update date_modified
    def pre_save_handler(sender, **kwargs):
        date_modified = datetime.datetime.today

    def __init__():
        pre_save.connect(pre_save_handler, sender=self)

or

class CommonData(models.Model):
    date_added = models.DateTimeField(default=datetime.datetime.today,null=False,blank=False)
    date_modified = models.DateTimeField(default=datetime.datetime.today,null=True,blank=True)

    # overriding save 
    def save(force_insert=False,force_update=False):
        date_modified = datetime.datetime.now
        return models.Model.save(force_insert, force_update)

I'm new to Django and Python and wondered which approach was more "django"? Which is more efficient? which is the "right" way to do this?

Answer 1

Since you're new to Django, you might find the Django Command Extensions useful:

http://code.google.com/p/django-command-extensions/

... which conveniently includes a TimeStampedModel you can derive your models from:

http://code.google.com/p/django-command-extensions/wiki/ModelExtensions

An abstract base class model that provides self-managed "created" and "modified" fields.

Answer 2

Did you try looking at DateTimeField's auto_now=True and auto_now_add=True? They do just what you need automatically. Otherwise, there is no real difference between doing save override and signal handling - in fact, pre_save signal is being called from django model's save method.

Docs: http://docs.djangoproject.com/en/dev/ref/models/fields/#datefield

Answer 3

You can define these in an Abstract Base Class and then inherit from that. It's sort of like having a MixIn that also defines model fields.

Answer 4

Note that auto_now_add and auto_now uses pre_save, which not working when bulk_create or update. Thus in your MySQL, for example, date_added field will be '0000-00-00 00:00:00' and it is possible to get warning: 'Warning: Column 'date_added' cannot be null'. So you can use auto_now* but you should be careful.