Keeping track of changes since the last save in django models

Question

A couple of times I've run into a situation, when at save time I need to know which model fields are going to be updated and act accordingly.

The most obvious solution to this is to take the primary key field and retrieve a copy of the model from the database:

class MyModel(models.Model):

    def save(self, force_insert=False, force_update=False, using=None):
        if self.id is not None:
            unsaved_copy = MyModel.objects.get(id=self.id)
            # Do your comparisons here
        super(MyModel, self).save(force_insert, force_update, using)

That works perfectly fine, however, it hits the database for every instance of the model you are saving (might be quite inconvenient if you are doing a lot of such saves).

It is obvious, that if one can "remember" the old field values at the start of model instance's lifetime (__init__), there should be no need to retrieve a copy of the model from the database. So I came up with this little hack:

class MyModel(models.Model):

    def __init__(self, *args, **kwargs):
        super(MyModel, self).__init__(*args, **kwargs)
        self.unsaved = {}
        for field in self._meta.fields:
            self.unsaved[field.name] = getattr(self, field.name, None)

    def save(self, force_insert=False, force_update=False, using=None):
        for name, value in self.unsaved.iteritems():
            print "Field:%s Old:%s New:%s" % (name, value, getattr(self, name, None))
        # old values can be accessed through the self.unsaved member
        super(MyModel, self).save(force_insert, force_update, using)

This seems to work, however it makes use of the non-public interface of django.db.models.Model.

Perhaps someone knows a cleaner way to do it?

Answer 1

I think your solution looks reasonable.

Alternatively you could have a Manager method called get_and_copy() (or something) that hung a copy of the original object off of what is returned. You could then use another Manager method, save_and_check() which took advantage of the copied original.

FWIW: If you are playing with contrib/admin templates there is a context variable called original which is a copy of the original object.

Update: I looked more closely at what admin is doing. In class ModelAdmin (located in django/contrib/admin/options.py) there is a method called construct_change_message(). It is being driven by formset.changed_data and formset.changed_objects, so django/forms/models.py class BaseModelFormSet is where the action is. See the method save_existing_objects(). Also look at the method _existing_object(). It's a little more complicated than what I mentioned before because they are dealing with the possibility of multiple objects, but they are basically caching the results of the query set on first access.