Automatically generating unique slugs in Django

unique-slugs.py

from django.db import models
from django.contrib.auth.models import User
from django.utils.text import slugify


class Post(models.Model):
    author = models.ForeignKey(
        User, related_name='post_author')

    title = models.CharField(max_length=200)
    slug = models.SlugField(max_length=200, unique=True)

    def _unique_slug(self):
        """
        return unique slug if origin slug is exist.
        eg: `foo-bar` => `foo-bar-1`
        """
        origin_slug = slugify(self.title)
        unique_slug = origin_slug
        numb = 1
        while Post.objects.filter(slug=unique_slug).exists():
            unique_slug = '%s-%d' % (origin_slug, numb)
            numb += 1
        return unique_slug

    def save(self, *args, **kwargs):
        if self.slug:  # edit mode
            if slugify(self.title) != self.slug:
                self.slug = self._unique_slug()
        else:  # create mode
            self.slug = self._unique_slug()
        super(Question, self).save(*args, **kwargs)