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 to previously (if they previously existed at all).

I'm thinking Pre-Save Signals, with a look-up to the original model doing a .objects.get(instance.id), but that feels wasteful. Also, has the validation already happened in pre_save()?

Answer 1

about model validation :

Note that full_clean() will not be called automatically when you call your model’s save() method

Then, about the pre-save signal, note that you get the instance that is being saved sent as a parameter with the message. As the former version of your model exists only in the database, I don't see where else you could get the previous value of the attributes ...

You don't tell why you want to do this so it's hard to say, but other solutions I'm thinking of right now :

* defining a custom signal that is sent everytime the attributes you are interested in are modified... This signal would then send two arguments : new value, old value
* perform the check directly when setting the attributes

If you give more details, it might be easier...

EDIT :

That's right ... If you emit a custom 'foo_has_updated', you will not be sure that the modification is saved.

In this case, I guess you could cache the variables that interest you while initializing the instance, and catch the post-save OR pre-save signal.

* With pre-save, you would be able to pre-process the data, but the saving operation might fail
* With post-save, you would be sure that the data has been saved.

Caching your variables could be done like this :

class CachedModel(models.Model):
    cached_vars = [var1, var2, varN]
    def __init__(self, *args, **kwargs):
        super(CachedModel, self).__init__(*args, **kwargs)
        self.var_cache = {}
        for var in self.cached_vars:
            self.var_cache[var] = copy.copy(getattr(self, var))

Or something like this ... Then, in your signal handler :

def post_save_handler(sender, **kwargs):
    instance = kwargs["instance"]
    [(instance.var_cache[var], getattr(instance, var)) for var in instance.cached_var]
    #[(<initial value>, <saved value>)

And you got what you needed (I think)!!!

Answer 2

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)