Setting default value for Foreign Key attribute

Question

What is the best way to set a default value for a foreign key field in a model? Suppose I have two models, Student and Exam with student having

Answer 1

In my case, I wanted to set the default to any existing instance of the related model. Because it's possible that the Exam with id 1 has been deleted, I've done the following:

class Student(models.Model):
    exam_taken = models.ForeignKey("Exam", blank=True)

    def save(self, *args, **kwargs):
        try:
            self.exam_taken
        except:
            self.exam_taken = Exam.objects.first()
        super().save(*args, **kwargs)

If exam_taken doesn't exist, django.db.models.fields.related_descriptors.RelatedObjectDoesNotExist will be raised when a attempting to access it.