class Viewed(models.Model):
    num_viewed = models.IntegerField(default=0)
    class Meta: abstract = True

    def inc_num_viewed(self, delta=1):
        # Atomic increment of the num_viewed.
        # Since this is an abstract model, Manager must be obtained dynamically.
        manager = type(self)._default_manager
        manager.filter(id=self.id).update(num_viewed=F('num_viewed') + delta)