class City(models.Model):
    name = models.CharField(max_length=200, unique=True, verbose_name='Название')
    slug = models.SlugField(max_length=220, verbose_name='Slug', blank=True)

    def __str__(self):
        return self.name

    def make_slug(self, instance_before):
        if not instance_before:
            self.slug = create_slug(self.name)

    def save(self, *args, **kwargs):
        instance_before = City.objects.get(pk=self.pk) if self.id else None

        self.make_slug(instance_before)

        super().save(*args, **kwargs)

def create_slug(val):
    return slugify(unidecode(val))


def unique_slug(model, slug, instance=None):
    orig = slug
    for x in itertools.count(1):
        if instance:
            if not model.objects.filter(slug=slug).exclude(pk=instance.pk).exists():
                break
        else:
            if not model.objects.filter(slug=slug).exists():
                break
        slug = '%s-%d' % (orig, x)
    return slug