AbstractBase(models.Model):
    foo = somefield

class Base(AbstractBase):
    c1 = models.OneToOneField('C1', null=True, blank=True)
    c2 = models.OneToOneField('C2', null=True, blank=True)

    def c(self):
       #only one additional query to get the whole set of data
        if self.c1_id:
            return self + c1
        if self.c2_id:
            return self + c2


class C1(models.Model):
     bar = somefield

class C2(models.Model):
     baz = somefield

bs = Base.objects.get(foo='foo')
bs.c.foo