Automatic redirects when changing slugs

file.py

from django.contrib.redirects.models import Redirect
from django.contrib.sites.models import Site

class BlogPost(models.Model):
    ...
    slug = models.CharField(unique=true, max_length=128)

    @models.permalink
    def get_absolute_url(def):
       return ('blogpost_detail', (), { 'slug' : self.slug })

from django.db.models.signals import pre_save
from signals import create_redirect
pre_save.connect(create_redirect, sender=BlogPost, dispatch_uid="001")

def create_redirect(sender, instance, **kwargs):
    if sender == BlogPost:
        try:
            o = sender.objects.get(id=instance.id)
            if o.slug != instance.slug:
                old_path = o.get_absolute_url()
                new_path = instance.get_absolute_url()
                # Update any existing redirects that are pointing to the old url
                for redirect in Redirect.objects.filter(new_path=old_path):
                    redirect.new_path = new_path
                    # If the updated redirect now points to itself, delete it
                    # (i.e. slug = A -> slug = B -> slug = A again)
                    if redirect.new_path == redirect.old_path:
                        redirect.delete()
                    else:
                        redirect.save()
                # Now add the new redirect
                Redirect.objects.create(
                                site=Site.objects.get_current(),
                                old_path=old_path,
                                new_path=new_path)
        except sender.DoesNotExist:
            pass