django - setting a non-nullable field in pre_save

Question

I'm trying to set a non nullable OneToOneField object (which hasn't been set) in the pre_save method.

class A(Model):
    b = models.OneToOneField(B, on_delete=CASCADE)

    @staticmethod
    def pre_save(sender, instance, **kwargs):
        try:
            instance.b
        except RelatedObjectDoesNotExist:
            instance.b = B()
            instance.b.save()

pre_save.connect(A.pre_save, A)

Even though the B object gets created and stored in the database, I get this error as if the A object doesn't point to B

django.db.utils.IntegrityError: NOT NULL constraint failed: app_a.b_id

If I set null=True in the field definition, of course I don't get the NOT NULL constraint failed error. But still, the A object doesn't store the reference to the B object.

I'm guessing this is because in the original A object there is no B reference, so when save is executed somehow it doesn't think it should save the b property.

Any ideas how to do this?

Seem like you want to create a reference to B when A just created right? So I think you can set it to Nullable, and create a reference to B by pre_save signal — Toan Quoc Ho, Sep 22 '19 at 01:34
I already tried that as stated in the last part of the question — Martin Massera, Sep 22 '19 at 01:39
instance.b, this won't raise any error when you set the field to be nullable. try `if not instance.b: #Create new B` instead. — Toan Quoc Ho, Sep 22 '19 at 01:48
That is not the problem. The problem is that I create the new B `if not instance.b: instance.b = B()` but after saving the `.b` property is still `None` (althouth the B got created) — Martin Massera, Sep 22 '19 at 13:38
Have you called `instance.save()` this one will commit all changes on instance. — Toan Quoc Ho, Sep 22 '19 at 13:59
Oh yeah, `pre_save`, I don't think we need to call `instance.save()`. It should save the reference @@ — Toan Quoc Ho, Sep 22 '19 at 14:25
Hi guys, I found the source of the problem, thanks everyone for thinking this with me — Martin Massera, Sep 22 '19 at 14:49

score 0 · Answer 1 · answered Sep 22 '19 at 14:48

0

So, I found the source of the problem!

This error happens when you assign the object before saving it. If you save the object and assign it later, it saves the object correctly:

b = B()
b.save()
instance.b = b

answered Sep 22 '19 at 14:48

Martin Massera

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django - setting a non-nullable field in pre_save

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