Django matching query does not exist after __dict__.copy()

Question

I'm making a copy of a model object and assigning it to another object model. Both models' attributes are the same. I do this to keep a historical copy of every change make to the records of the first. This is a simplified version of the code:

hm = HistoricModel()
hm.__dict__ = obj.__dict__.copy()
hm.save()

In order to handle the object's historic relations, I override the save method of the historic object and update the relation fields:

def save(self, *args, **kwargs):

    self.relation = HistoricRelation.objects.filter(id=self.relation.id).latest()
    super(HistoricModel, self).save(*args, **kwargs)

When running the code I get (SOMETIMES!):

HistoricRelation matching query does not exist.

After much debugging if realized that the error comes from

self.relation.id 

If I make a deep copy of the object instead:

hm.__dict__ = copy.deepcopy(obj.__dict__)

then it works!

I have the simple copy version (as in the beginning) in an AWS Linux server and it works just fine. In my Windows 8.1 laptop, however, it doesn't. I'm using Python 2.7 and Django 1.6 in both places.

So questions: Why is this? Is it related to the OS? is there an alternative to the deepcopy? I'm concerned about the performance impact it might have.