Unique BooleanField value in Django?

Question

Suppose my models.py is like so:

class Character(models.Model):
    name = models.CharField(max_length=255)
    is_the_chosen_one = models.BooleanField()

I want only one of my Character instances to have is_the_chosen_one == True and all others to have is_the_chosen_one == False . How can I best ensure this uniqueness constraint is respected?

Top marks to answers that take into account the importance of respecting the constraint at the database, model and (admin) form levels!

Answer 1

Whenever I've needed to accomplish this task, what I've done is override the save method for the model and have it check if any other model has the flag already set (and turn it off).

class Character(models.Model):
    name = models.CharField(max_length=255)
    is_the_chosen_one = models.BooleanField()

    def save(self, *args, **kwargs):
        if self.is_the_chosen_one:
            try:
                temp = Character.objects.get(is_the_chosen_one=True)
                if self != temp:
                    temp.is_the_chosen_one = False
                    temp.save()
            except Character.DoesNotExist:
                pass
        super(Character, self).save(*args, **kwargs)