from functools import wraps

def unique_boolean(field):
    def factory(func):
        @wraps(func)
        def decorator(self):
            if getattr(self, field):
                try:
                    tmp = self.__class__.objects.get(**{ field: True })
                    if self != tmp:
                        setattr(tmp, field, False)
                        tmp.save()
                except self.__class__.DoesNotExist:
                    pass
            return func(self)
        return decorator
    return factory

# Usage:
class MyDefaultUniqueModel(models.Model):
    default = models.BooleanField()

    @unique_boolean('default')
    def save(self):
        super(MyDefaultUniqueModel, self).save()