[Fixed]-Django – Check diference between old and new value when overriding save method

You can store the old value inside the init method:

def __init__(self, *args, **kwargs):
    super(MyModel, self).__init__(*args, **kwargs)
    self.old_my_field = self.my_field

def save(self, *args, **kwargs):
    print self.old_my_field
    print self.my_field

You can probably use deepcopy or something alike to copy the whole object for later use in the save and delete methods.

Django doesn’t cache the old values of the model instance, so you need to do that yourself or perform another query before save.

One common pattern is to use a pre-save signal (or put this code directly in your save() method, as you’ve done):

old_instance = MyModel.objects.get(pk=instance.pk)
# compare instance with old_instance, and maybe decide whether to continue

If you want to keep a cache of the old values, then you would probably do that in your view code:

from copy import deepcopy
object = MyModel.objects.get(pk=some_value)
cache = deepcopy(object)

# Do something with object, and then compare with cache before saving

There was a recent discussion on django-developers about this as well, with some other possible solutions.

I am checking the difference to old values using a django-reversion signal, but the same logic would apply to the save signals. The difference for me being that I want to save whether the field was saved or not.

@receiver(reversion.pre_revision_commit)
def it_worked(sender, **kwargs):
    currentVersion = kwargs.pop('versions')[0].field_dict
    fieldList = currentVersion.keys()
    fieldList.remove('id')
    commentDict = {}
    print fieldList
    try:
        pastVersion = reversion.get_for_object(kwargs.pop('instances')[0])[0].field_dict
    except IndexError:
        for field in fieldList:
            commentDict[field] = "Created"
        comment = commentDict
    except TypeError:
        for field in fieldList:
            commentDict[field] = "Deleted"
        comment = commentDict
    else:
        for field in fieldList:
            try:
                pastTest = pastVersion[field]
            except KeyError:
                commentDict[field] = "Created"
            else:       
                if currentVersion[field] != pastTest:
                    commentDict[field] = "Changed"
                else:
                    commentDict[field] = "Unchanged"
        comment = commentDict
    revision = kwargs.pop('revision')
    revision.comment = comment
    revision.save()
    kwargs['revision'] = revision
    sender.save_revision