It's common for me to want a unique slug based on a user supplied name of an object. Following the pattern outlined at ​djang-tips-auto-populated-fields, it's easy to get the field populated without user intervention. However, by setting the slug field in the save() method the field isn't validated and thus unique=True restrictions aren't checked. I wrote the following code to attempt to pick slugs which are unique within a particular model. It's certainly not ideal (it suffers from race condition as well as potential for pounding a database with lots of small queries until it finds a good slug field), but it works for my small/mid-sized apps.

Feel free to suggest improvements to 'wam-djangowiki spam@…' (remove the ' spam' from the address there).

def SlugifyUniquely(value, model, slugfield="slug"):
        """Returns a slug on a name which is unique within a model's table

        This code suffers a race condition between when a unique
        slug is determined and when the object with that slug is saved.
        It's also not exactly database friendly if there is a high
        likelyhood of common slugs being attempted.

        A good usage pattern for this code would be to add a custom save()
        method to a model with a slug field along the lines of:

                from django.template.defaultfilters import slugify

                def save(self):
                    if not self.id:
                        # replace self.name with your prepopulate_from field
                        self.slug = SlugifyUniquely(self.name, self.__class__)
                super(self.__class__, self).save()

        Original pattern discussed at
        http://www.b-list.org/weblog/2006/11/02/django-tips-auto-populated-fields
        """
        suffix = 0
        potential = base = slugify(value)
        while True:
                if suffix:
                        potential = "-".join([base, str(suffix)])
                
                if not model.objects.filter(**{slugfield: potential}).count():
                        return potential
                # we hit a conflicting slug, so bump the suffix & try again
                suffix += 1