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

I would modify @vault's answer above slightly (this may be a new feature). It is definitely desirable to refer to the field by a natural name. However instead of overriding the Manager I would simply use the to_field param of ForeignKey:

class Country(models.Model):
    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, to_field='sigla', default='IT')