Python-сообщество

class Forum(models.Model):
    name = models.CharField(maxlength=255)

class Topic(models.Model):
    forum = models.ForeignKey(Forum)
    subject = models.CharField(maxlength=255)

class Post(models.Model):
    topic = models.ForeignKey(Topic)
    body  = models.TextField()

class Forum(models.Model):
    name = models.CharField(maxlength=255)

class Topic(models.Model):
    forum = models.ForeignKey(Forum, related_name="topics")
    subject = models.CharField(maxlength=255)

class Post(models.Model):
    topic = models.ForeignKey(Topic, related_name="posts")
    body  = models.TextField()

forum = Forum.objects.get(pk=1)

forum.topics.all() # все темы данного форума

topic = forum.topics.all()[0]

topic.posts.all() # все посты данной темы