docs: def unique_slugify(instance, value, slug_field_name='slug', queryset=None,
                     slug_separator='-'):
      """
      Calculates and stores a unique slug of ``value`` for an instance.
  
      ``slug_field_name`` should be a string matching the name of the field to
      store the slug in (and the field to check against for uniqueness).
  
      ``queryset`` usually doesn't need to be explicitly provided - it'll default
      to using the ``.all()`` queryset from the model's default manager.
      """
      slug_field = instance._meta.get_field(slug_field_name)
  
      slug = getattr(instance, slug_field.attname)
      slug_len = slug_field.max_length
  
      # Sort out the initial slug, limiting its length if necessary.
      slug = slugify(value)
      if slug_len:
          slug = slug[:slug_len]
      slug = _slug_strip(slug, slug_separator)
      original_slug = slug
  
      # Create the queryset if one wasn't explicitly provided and exclude the
      # current instance from the queryset.
      if queryset is None:
          queryset = instance.__class__._default_manager.all()
      if instance.pk:
          queryset = queryset.exclude(pk=instance.pk)
  
      # Find a unique slug. If one matches, at '-2' to the end and try again
      # (then '-3', etc).
      next = 2
      while not slug or queryset.filter(**{slug_field_name: slug}):
          slug = original_slug
          end = '%s%s' % (slug_separator, next)
          if slug_len and len(slug) + len(end) > slug_len:
              slug = slug[:slug_len-len(end)]
              slug = _slug_strip(slug, slug_separator)
          slug = '%s%s' % (slug, end)
          next += 1
  
      setattr(instance, slug_field.attname, slug)