How to validate uniqueness constraint across foreign key (django)

Question

I have the following (simplified) data structure:

Site
-> Zone
   -> Room
      -> name

I want the name of each Room to be unique

Answer 1

class Room(models.Model):
    zone = models.ForeignKey(Zone)
    name = models.CharField(max_length=255)

    def validate_unique(self, *args, **kwargs):
        super(Room, self).validate_unique(*args, **kwargs)
        qs = Room.objects.filter(name=self.name)
        if qs.filter(zone__site=self.zone__site).exists():
            raise ValidationError({'name':['Name must be unique per site',]})

I needed to make similar program. It worked.