Django: Before a model is updated, I'd like to “look at” its previous attributes

Question

When an update/create is performed on a Django model (.save()) I would like to be able to \"step in\" and compare some particular attributes to what they were set t

Answer 1

You can use the pre_save signal and compare the db record (the old version) with the instance record (the updated, but not saved in the db version).

Take a model like this one as example:

class Person(models.Model):
    Name = models.CharField(max_length=200)

In a pre_save function you can compare the instance version with the db version.

def check_person_before_saving(sender, **kwargs):
    person_instance = kwargs['instance']
    if person_instance.id:
        # you are saving a Person that is already on the db
        # now you can get the db old version of Person before the updating

        # you should wrap this code on a try/except (just in case)
        person_db = Person.objects.get(id=person_instance.id)

        # do your compares between person_db and person_instance
        ...

# connect the signal to the function. You can use a decorator if you prefer
pre_save.connect(check_person_before_saving, sender=Person)