In Django, for a OneToOne relationship between two objects of the same class, can an object be prevented from referring to itself at the db level?

Question

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)

As it is, under the Django Admin site, I’m able to select University A as the sister_university of University A (itself). Is it possible to enforce some sort of a rule at the database level so a university object can never be its own sister_university?

Alternatively, is there a better way of accomplishing what I’m trying to do?

@BearBrown They’re two different questions. Same example code but the previous question asks whether *uniqueness* can be assured and the second question (this question) asks if I can prevent an object from referring to itself. So please don’t close this. Thanks! — Joe, May 15 '18 at 20:37
you just added info about admin but unique for sister is present in two question — Brown Bear, May 15 '18 at 20:42
@BearBrown Maybe, I’m missing something. Can you please point me to where this question says “unique for sister”? I’d be happy to correct it. — Joe, May 15 '18 at 20:47
@BearBrown The main point of this question is preventing an object from referring to itself, which is not an issue raised in the previous question. — Joe, May 15 '18 at 20:48
to exclude the conflict situation in next time, look on example in the top of the question: [statement-to-django-queryset](https://stackoverflow.com/questions/50286057/how-can-i-convert-this-sql-statement-to-django-queryset) — Brown Bear, May 15 '18 at 21:02
@BearBrown That’s helpful. I’ll be sure to do that next time. Thanks — Joe, May 15 '18 at 21:06

score 0 · Answer 1 · answered May 15 '18 at 21:21

0

What you want is a check constraint, something like:

alter table university
add constraint chk_non_self_ref check (id <> sister_university_id);

I'm not aware of a way to define this in Django models (but I'm guessing you can add it to a migration).

answered May 15 '18 at 21:21

thebjorn

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score 0 · Accepted Answer · answered May 16 '18 at 06:00

At the model level, you can override the clean() method of your model to implement that check every time a University instance is saved.

def clean(self):

    if self.sister_university.id == self.id:
        raise ValidationError('A university object can never be its own sister_university.')

In Django, for a OneToOne relationship between two objects of the same class, can an object be prevented from referring to itself at the db level?

2 Answers2