class AddressBookAttribute(models.Model):
    attribute = models.CharField(max_length=64, blank=False, null=False, unique=True, verbose_name=_("Attribute"))
    available_for_group = models.ForeignKey(Group, on_delete=models.SET_NULL, null=True, blank=True)
    visible = models.BooleanField(verbose_name=_('Visible'), blank=False, null=False, default=True)
    sorting_index = models.PositiveIntegerField(verbose_name=_('Sort Index'), validators=[MinValueValidator(1)])