problème models avec les foreign key

# -*- coding: utf-8 -*-
from django.db import models
from miraemos.registration.models import Joueur

TYPE_JOUEUR=(
    (0,'tous'),
    (1,'soldat'),
    (2,'ingénieur')
)
       
class BrancheMission:
    bg          = models.IntegerField("borne gauche")
    bd          = models.IntegerField("borne droite")
    nombreRequis = models.IntegerField("nombre de sous-mission requise pour finir la mission")
    ordre        = models.BooleanField("sous-mission ordonnée ou non")
   
class Mission:
    idBrancheFeuille    = models.ForeignKey(BrancheMission, verbose_name="id de la branche")
    nom          = models.CharField("nom", max_length=50)
    description  = models.TextField("description", max_length=500)
    image        = models.ImageField("image de la mission",upload_to='mission/vignette/')
    xprequise    = models.IntegerField("expérience requise pour débloquer la mission")
    typeJoueur  = models.IntegerField("type de joueur requis pour la mission",choices=TYPE_JOUEUR)
    def __unicode_(self):
        return u'%s' % self.nom

Traceback (most recent call last):
  File "manage.py", line 11, in <module>
    execute_manager(settings)
  File "/usr/lib/python2.5/site-packages/django/core/management/__init__.py", line 301, in execute_manager
    utility.execute()
  File "/usr/lib/python2.5/site-packages/django/core/management/__init__.py", line 248, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/lib/python2.5/site-packages/django/core/management/base.py", line 77, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/usr/lib/python2.5/site-packages/django/core/management/base.py", line 90, in execute
    self.validate()
  File "/usr/lib/python2.5/site-packages/django/core/management/base.py", line 117, in validate
    num_errors = get_validation_errors(s, app)
  File "/usr/lib/python2.5/site-packages/django/core/management/validation.py", line 28, in get_validation_errors
    for (app_name, error) in get_app_errors().items():
  File "/usr/lib/python2.5/site-packages/django/db/models/loading.py", line 128, in get_app_errors
    self._populate()
  File "/usr/lib/python2.5/site-packages/django/db/models/loading.py", line 57, in _populate
    self.load_app(app_name, True)
  File "/usr/lib/python2.5/site-packages/django/db/models/loading.py", line 72, in load_app
    mod = __import__(app_name, {}, {}, ['models'])
  File "/home/barre/workspace/miraemos/miraemos/../miraemos/missions/models.py", line 17, in <module>
    class Mission:
  File "/home/barre/workspace/miraemos/miraemos/../miraemos/missions/models.py", line 18, in Mission
    idBrancheFeuille    = models.ForeignKey(BrancheMission, verbose_name="id de la branche")
  File "/usr/lib/python2.5/site-packages/django/utils/maxlength.py", line 47, in inner
    func(self, *args, **kwargs)
  File "/usr/lib/python2.5/site-packages/django/db/models/fields/related.py", line 604, in __init__
    assert isinstance(to, basestring), "%s(%r) is invalid. First parameter to ForeignKey must be either a model, a model name, or the string %r" % (self.__class__.__name__, to, RECURSIVE_RELATIONSHIP_CONSTANT)
AssertionError: ForeignKey(<class miraemos.missions.models.BrancheMission at 0x87dac8c>) is invalid. First parameter to ForeignKey must be either a model, a model name, or the string 'self'

class BrancheMission(models.Model):
    bg          = models.IntegerField("borne gauche")
    bd          = models.IntegerField("borne droite")
    nombreRequis = models.IntegerField("nombre de sous-mission requise pour finir la mission")
    ordre        = models.BooleanField("sous-mission ordonnée ou non")

class Mission(models.Model):
    idBrancheFeuille    = models.ForeignKey(BrancheMission, verbose_name="id de la branche")
    nom          = models.CharField("nom", max_length=50)
    description  = models.TextField("description", max_length=500)
    image        = models.ImageField("image de la mission",upload_to='mission/vignette/')
    xprequise    = models.IntegerField("expérience requise pour débloquer la mission")
    typeJoueur  = models.IntegerField("type de joueur requis pour la mission",choices=TYPE_JOUEUR)
    def __unicode_(self):
        return u'%s' % self.nom