app.model: 'fieldname' is a manually-defined m2m relation through model Zzzzz, which does not have foreign keys to XXX and YYY

django1.5 でmanytomany field で中間テーブルを使うときに出たエラー

class Group(models.Model):
      name = models.CharField(max_length=40)
      user = models.ManyToManyField(User, through='Staff')

      def __unicode__(self):
          return self.name

  class Staff(models.Model):
      name = models.ForeignKey(User, related_name='staffname')

      def __unicode__(self):
          return self.name

これでsyncdbしたら、次のようなエラーが

app.groups: 'user' is a manually-defined m2m relation through model Staff, which does not have foreign keys to User and Group

とのことなので、Staff のモデルにはUserの外部キーは持っているが、groupの外部キーがなかったので追加する。

  class Staff(models.Model):
      name = models.ForeignKey(User, related_name='staffname')
      group = models.ForeignKey(Group, related_name='staffname')

      def __unicode__(self):
          return self.name