[Django] OneToOneField with primary key bug?

gistfile1.py

from django.db import models


class Event(models.Model):
    pass


class EventContent(models.Model):
    event = models.OneToOneField(Event, primary_key=True, related_name='%(class)s')
    type = 'default_event'

    class Meta:
        abstract = True

    def __unicode__(self):
        return u'Event #%d [%s]' % (self.type, self.event.id)

    def save(self, force_insert=False, force_update=False, using=None):
        if not hasattr(self, 'event'):
            self.event = Event()
            self.event.save()
        return super(EventContent, self).save(force_insert, force_update, using)


class EventType1(EventContent):
    type = 'type1_event'


class EventType2(EventContent):
    type = 'type2_event'


e1 = EventType1()
e1.save()
print e1.event.id # 1
print EventType1.objects.all() # [<EventType1: Event #1 [type1_event]>]
e2 = EventType2()
e2.save() 
print e2.event.id # 2
print EventType2.objects.all() # [<EventType2: Event #1 [type2_event]>]