I'm trying to create a simple model to keep track of discount coupons in Django 1.10 (with Postgres 9.5 as underlying database) and I was wondering if there's a way to make sure that a coupon instance (id, perhaps is a more accurate term?) doesn't appear in two M2M relationships at the same time.

I'm sure everyone is familiar with how discount coupons work but, just in case, let me explain my use case:

1. Some coupons would be always applied. For instance: "Free delivery in your first purchase", or "10% off Pepsi for the rest of your life"... things like that.
2. Some other coupons would be applied through a code (a simple string, really) that the user would have to input somewhere (like "Get a 5% off with the code "5-OFF" "... yeah, I'll probably have to work on the obfuscation of the codes :-D )
3. The user could say "No, I don't want to apply this coupon to this order, I'll use it later". For instance: if the user can use a one-time 5% off coupon, but wants to keep it for a large purchase. Let's say the customer knows he's going to make a large purchase in the upcoming future, and right now he's doing a small one. He might wanna keep the 5% off for the later (bigger) purchase.

To keep track of those things, I have a model like this:

class CouponTracker(models.Model):
    # Coupons that require a code to be activated:
    extra_applied_coupons = ManyToManyField('Coupon', related_name='+')

    # Coupons that the user could have applied and specifically
    # said no (such as 5% off if the purchase is small, and wants
    # to use it later):
    vetoed_coupons = ManyToManyField('Coupon', related_name='+')

So, the question is:

How can I enforce (at a database level, through a constraint) that a coupon does not appear at the same time in extra_applied_coupons and vetoed_coupons?

Thank you in advance!