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

Here's my idea: play around with properties.

Say you have this class:

class Foo(models.Model):
    name = models.CharField()

Instead, rename your field (you won't need a migration if it's the first time you're doing this) and:

class Foo(models.Model):
    _name = models.CharField()

    @property
    def name(self):
        return self._name

    @name.setter
    def name(self, new_value):
        if not getattr(self, '_initial_name', False):
            self._initial_name = self._name

        if new_value != self._initial_name:
            self._name_changed = True
        else:
            self._name_changed = False

        self._name = new_value

We added two attributes to your Foo instances: '_initial_name' and '_name_changed' and a property: 'name'. These are not model fields and will never be saved to the database. Also, you won't have to mess with the '_name' field any longer as long as the 'name' property takes care of everything.

Now, your 'pre_save' or 'post_save' signal handler can make checks on what has changed:

def handle_pre_save(sender, **kwargs):
    foo = kwargs['instance']
    if getattr(foo, '_name_changed', False):
        log.debug("foo changed its name from '%s' to '%s'",
                  foo._initial_name, foo.name)