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 both of your examples, you're hard-coding the id of the default instance. If that's inevitable, I'd just set a constant.

DEFAULT_EXAM_ID = 1
class Student(models.Model):
    ...
    exam_taken = models.ForeignKey("Exam", default=DEFAULT_EXAM_ID)

Less code, and naming the constant makes it more readable.

Answer 2

I use natural keys to adopt a more natural approach:

<app>/models.py

from django.db import models

class CountryManager(models.Manager):
    """Enable fixtures using self.sigla instead of `id`"""

    def get_by_natural_key(self, sigla):
        return self.get(sigla=sigla)

class Country(models.Model):
    objects = CountryManager()
    sigla   = models.CharField(max_length=5, unique=True)

    def __unicode__(self):
        return u'%s' % self.sigla

class City(models.Model):
    nome   = models.CharField(max_length=64, unique=True)
    nation = models.ForeignKey(Country, default='IT')