Django: How to access original (unmodified) instance in post_save signal

Question

I want to do a data denormalization for better performance, and put a sum of votes my blog post receives inside Post model:

class Post(models.Model):
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Answer 1

I believe post_save is too late to retrieve the unmodified version. As the name implies the data has already been written to the db at that point. You should use pre_save instead. In that case you can retrieve the model from the db via pk: old = Vote.objects.get(pk=instance.pk) and check for differences in the current instance and the previous instance.