通過反向排序ForeignKey DateTimeField？

Question

考慮這兩款車型：

class pm_thread(models.Model): 
    subject = models.CharField(max_length=200) 
    participants = models.ManyToManyField(User) 

    def latest_message(self): 
     message = self.message_set.order_by('-datetime') 
     if message: 
      return message[0] 
     return None 

class pm_message(models.Model): 
    thread = models.ForeignKey(pm_thread, related_name='message_set') 
    user = models.ForeignKey(User, related_name='sent_message') 
    datetime = models.DateTimeField(auto_now_add=True) 
    content = models.TextField() 

我怎麼能爲了一個pm_thread查找的最新消息的日期？例如：

threads = pm_thread.objects.filter(participants=request.user).order_by(latest_message.datetime) 

Answer 1

嘗試：

threads = pm_thread.objects.filter(participants=request.user).order_by('-pm_message__datetime')