I have a OneToOne relationship between two objects of the same class in a Django app. Is it possible to enforce the uniqueness of this relationship?

I think that what you need is to make this relationship symmetric.

You can accomplish this by overriding the save() method of the University model:

def save(self, *args, **kwargs):    super(University, self).save()    if self.sister_university:        self.sister_university.sister_university = self

I never did this kind of things, but I think that you can make this process through this way :

Method : unique_together()

You can use Options.unique_together and set your university_A and university_B as a unique pair.

unique_together = ("university_A", "university_B")

In your models.py file, you should have something like this (with maybe some issues, but the idea is there) :

class University(models.Model):    ...    university = models.ForeignKey('self', on_delete=models.CASCADE)    sister_university = models.ForeignKey('self', on_delete=models.CASCADE)    class Meta:        unique_together     = (('university','sister_university'),)

You should find precious details there : https://docs.djangoproject.com/en/2.0/ref/models/options/

I never tried this command, but it seems to solve your issue according to your context.