forward declaration

Hallo,

How could I implement forward declaration in django models?
this example wouldn't work for me:
Has anyone an idear or a solution?

This django model show a simple implementation for a bloodline app!
********************************************************************************
from django.db import models
from django.contrib.auth.models import User
from django.contrib import admin

class Parent:
    pass
class Child:
    pass

class AbstractUser(models.Model):
    user_id = models.ForeignKey(User, blank=True)
    birthday = models.DateField("birthday", blank=True, null=True)
    day_of_death = models.DateField("day of death", blank=True,
null=True)
    parents = models.ManyToManyField(Parent, blank=True, null=True)
    childs = models.ManyToManyField(Child, blank=True, null=True)

class Parent(AbstractUser):
    #user_id = models.ForeignKey(User, blank=True)
    def __unicode__(self):
        return "%s %s" % (self.user_id.first_name,
self.user_id.last_name)

class Child(AbstractUser):
    #user_id = models.ForeignKey(User, blank=True)
    def __unicode__(self):
        return "%s %s" % (self.user_id.first_name,
self.user_id.last_name)

class MyUser(AbstractUser):
    """ Represent a User for this site
    """
    def __unicode__(self):
        return "%s %s" % (self.user_id.first_name,
self.user_id.last_name)

admin.site.register(MyUser)
admin.site.register(Parent)
admin.site.register(Child)
********************************************************************************
I got this error message:
AssertionError: ManyToManyField(<class
stammbaum.bloodline.models.Parent at 0x84fc11c>) is invalid. First
parameter to ManyToManyField must be either a model, a model name, or
the string 'self'

regards
R.Klain
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