auto_now using signals 

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# Takes model and fieldname and sets field to current datetime on pre_save
def auto_now_on_save(model, fieldname):
	from django.db.models import signals
	from django.dispatch import dispatcher
	def _update_datefield(instance):
		from datetime import datetime
		setattr(instance, fieldname, datetime.now())
	dispatcher.connect(_update_datefield, signal=signals.pre_save, sender=model, weak=False)

Comments

ericflo (on July 10, 2008):

It's good to know that for now that Django's signals are a fairly large performance hit. To remedy this, you can simply do this: 

def save(self):
    self.myfield = datetime.now()
    super(MyModel, self).save()

It's one more line than if you import your solution and apply it to a field, but doesn't incur the performance penalty that signals currently entail.

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