Question

I have a model with three classes: PendingAuthorisation, Director, and Member. Director and Member instances are considered inactive if they have a corresponding PendingAuthorisation.

My problem is quite how to model this. I would ideally like PendingAuthorisation to have just one field, defers, which can refer to either a Director or a Member. If I create a foreign key in both Director and Member then I need to have two differently-named relations, and when using a PendingAuthorisation I would need to check both to find the object it is deferring. In no case should a PendingAuthorisation be deferring one object of each type.

Any suggestions on how to model this?

Answer 1

I’d recommend having two foreign keys (with a sanity check in your save method to make sure that they’re not both set), and then a property to return back the object that’s set.

Think about something like:

from django.db import models

class PendingAuthorization(models.Model):
    director = models.ForeignKey(Director, null=True, blank=True)
    member = models.ForeignKey(Member, null=True, blank=True)

    def save(self, *args, **kwargs):
        if self.director and self.member:
            raise ValueError, 'Both a director and member are set; this is not allowed.'
        return super(PendingAuthorization, self).save(*args, **kwargs)

    @property
    def defers(self):
        if self.director:
            return self.director
        if self.member:
            return self.member
        return None