I have the following in my app:
class University(models.Model):
...
sister_university = models.OneToOneField('self', related_name =
'university_sister_university',
blank=True, null=True,
on_delete=models.SET_NULL)
I only want a university to be related to one other university in both directions of that relationship.
For example, in the database, if I select university A as the sister_university of university B, I only want to be allowed to select university B as the sister_university under university A also. However, as it is, that second relationship is not enforced.
For example: Right now, under the Django Admin site, if I first select university A as the sister university of university B, I am still able to select any other university as the sister university of the university A object. I'm not constrained to only selecting university B.
Is it possible to enforce that uniqueness at the database level? Is there a better way of accomplishing what I'm trying to do?
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 :
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.
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