Question

I’ve got a model which, when saved, should create and save one of its foreign keys if it doesn’t already exist:

class MyModel(m.Model):
    fk = m.ForeignKey(AnotherModel)

    def save(self):
        if not self.fk_id:
            self.fk = AnotherModel()
            self.fk.save()
        return super(MyModel, self).save()

But the call to super(MyModel, self).save() raises an error, because self.fk_id is still null:

Traceback (most recent call last):
  ...
  File ".../models.py", line 10, in save
    return super(MyModel, self).save()
  ...
  File ".../django/db/backends/postgresql_psycopg2/base.py", line 44, in execute
    return self.cursor.execute(query, args)
IntegrityError: null value in column "fk_id" violates not-null constraint

Note: a debugger shows that self.fk.id has been set correctly, and everything seems to work when self.fk_id = self.fk.id is added.

So, my question: why is self.fk_id = self.fk.id necessary? And is it possible to avoid doing that explicitly?

Answer 1

I think this bug report is your answer. Currently, you just need to set the related object after it has a primary key itself. They’re still discussing the patch.