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 exam_taken as foreign key. How would I ideally set a default value for it? Here's a log of my effort

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

Works, but have a hunch there's a better way.

def get_exam():     return Exam.objects.get(id=1)  class Student(models.Model):     ....     .....     exam_taken = models.ForeignKey("Exam", default=get_exam) 

But this fails with tables does not exist error while syncing.

Any help would be appreciated.

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') 

Answer 2

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.