class Relationship(models.Model):
    person1 = models.ForeignKey(User, on_delete=models.CASCADE, related_name="person1")
    person2 = models.ForeignKey(User, on_delete=models.CASCADE, related_name="person2")
    notification = models.OneToOneField('Notification', null=True, blank=True, on_delete=models.SET_NULL)

    class Meta:
        unique_together = (('person1', 'person2'),)

    def __str__(self):
        return '%s with %s' % (self.person1.get_username(), self.person2.get_username())