How to set only one of model's objects to default ?

I would like to setup Coupons for my store. I have class Coupon.
I would like to be able to define ONE default Coupon which will be applied in case there are no coupons applied yet to the cart.
So, how can I set only one of the coupons to be the default one ?
What approach would you suggest ?

FYI: I will manage Coupons using Django Admin.

My current Coupon class looks like this:

class Coupon(models.Model):
    code = models.CharField(max_length=10,unique=True)
    description = models.TextField(blank=True, null=True)
    valid_from = models.DateTimeField(
    valid_to = models.DateTimeField(

I don’t fully understand what you’re trying to do but maybe a One To One relation will be helpful.

In a way it depends upon what you mean by “Applied”. Is there another model that will be associated with a Coupon? If so, you may want to set the association on that other model to your default as a default.