constraint in Django: exactly 1 of 2 foreign keys has to be null

Question

I'd like to allow null on two foreign key fields.

But the two fields should not be null at the same time.
Actually, exactly one has to be set at any time.

How can I express this?

The two tables the foreign keys reference are not the same.

Somewhat related: https://stackoverflow.com/questions/6928692/how-to-express-a-one-to-many-relationship-in-django — Sergey Orshanskiy, Nov 11 '18 at 21:16
This is frequently a design anti-pattern for subtyping. [How can you represent inheritance in a database?](https://stackoverflow.com/q/3579079/3404097) — philipxy, Nov 11 '18 at 21:23

score 5 · Accepted Answer · answered Jul 12 '13 at 06:02

5

You can't achive this by adding something on model fields. You will have to put this logic in your save().

class MyModel(models.Model):
    fk1 = models.ForeignKey(Some, null=True)
    fk2 = models.ForeignKey(Other, null=True)

    def save(self, *args, **kwargs):
       if not fk1 and not fk2:
           raise Exception("You can't leave both fields as null")
       super(self, MyModel).save(*args, **kwargs)

answered Jul 12 '13 at 06:02

Akshar Raaj

14,231
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Shouldn't validation be done in the `clean` method? – schlumpfpirat Sep 28 '20 at 12:35

score 0 · Answer 2 · answered Aug 27 '18 at 08:00

0

You can use the following method :

Class TestModel(models.Model):
   forgein_key_1 = models.ForeignKey(YourModel)
   forgein_key_2 = models.ForeignKey(YourOtherModel)

and override the save method as:

    def save(self, *args, **kargs):
        if not (self.forgein_key_1 or self.forgein_key_2):
          raise Exception("Your Custom Exception Message here")
        super(self, TestModel).save(*args, **kargs)

answered Aug 27 '18 at 08:00

Kiran Karandikar

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2

Probably an explanation would help as well. – scopchanov Aug 27 '18 at 08:10

constraint in Django: exactly 1 of 2 foreign keys has to be null

2 Answers2