Django Admin: Detect if a subset of an object fields has changed and which of them

Question

I need to detect when some of the fields of certain model have changed in the admin, to later send notifications depending on which fields changed and previous/current values of those fields.

I tried using a ModelForm and overriding the save() method, but the form's self.cleaned_data and seld.instance already have the new values of the fields.

Answer 1

What you'll need to do is get an extra copy of the object you're working on from the database inside the save method before fully saving it. Example:

class MyModel(models.Model):
    field1 = models.CharField(max_length=50)

    def save(self):
        if self.id:
            try:
                old = MyModel.objects.get(pk=self.id)
                if old.field1 != self.field1:
                    # Process somehow
            except MyModel.DoesNotExist:
                pass
        super(MyModel, self).save()

Answer 2

In order to get differences of two model instances, you can also use this function. It compare to model instances and returns dictionary of changes.

Answer 3

To avoid extra DB lookup, I modified constructor to remember initial value and use this in save method later:

class Package(models.Model):
    feedback = models.IntegerField(default = 0, choices = FEEDBACK_CHOICES)
    feedback_time = models.DateTimeField(null = True)

    def __init__(self, *args, **kw):
        super(Package, self).__init__(*args, **kw)
        self._old_feedback = self.feedback

    def save(self, force_insert=False, force_update=False, *args, **kwargs):
        if not force_insert and self.feedback != self._old_feedback:
            self.feedback_time = datetime.utcnow()
        return super(Package, self).save(force_insert, force_update, *args, **kwargs)