Takie parametry mają używane tu pola:
 id = models.AutoField(primary_key=True)
 title = models.CharField(max_length=120)
 slug = models.SlugField(unique=True, blank=True)

 def product_pre_save_receiver(sender, instance, *args, **kwargs):
    last_id = sender.objects.all().last().id
    instance.id = last_id + 1
    new_slug = create_slug(instance)                            # 1
    if not instance.slug or instance.slug != new_slug:    # 2
       instance.slug = new_slug                                     # 3

tak wygląda create_slug

 def create_slug(instance, new_slug=None):
    if new_slug:
       slug = new_slug
    else:
        slug = slugify(instance.title)

    slug_exists = Product.objects.filter(slug=slug).exists()
    if slug_exists:
        new_slug = "%s-%s" % (slug, instance.id)
        return create_slug(instance, new_slug=new_slug)
    return slug