title: "<Message: title>" needs to have a value for field "id" before this many-to-many relationship can be used.
body: <p>I have a Message model, And in it I have <code>create_user</code> ForeignKey and <code>receive_user</code> ManyToManyField:</p>
  
  <pre><code>class Message(models.Model):
      """
      消息
      """
      message_num = models.CharField(default=getMessageNum, max_length=16, help_text="消息") # 注意：message_num 相同，说明是同一次发送
  
      title = models.CharField(max_length=64, help_text="消息名称")
      content = models.CharField(max_length=1024, help_text="消息内容")
  
      create_user = models.ForeignKey(User, related_name="created_messages",help_text="创建者")
      receive_user = models.ManyToManyField(User, related_name="received_messages", help_text="接受者")
  
  
      def __str__(self):
          return self.title
      def __unicode__(self):
          return self.title
  </code></pre>
  
  <p>When I use the bellow to save a message, I except a exception:</p>
  
  <pre><code>try:
      receive_user = User.objects.get(id=user_id)
      message = Message.objects.create(
          title=title,
          content=content,
          create_user=create_user,
          receive_user=receive_user,
      )
      message.save()
  except Exception as e:
      raise e
  </code></pre>
  
  <p>I get the exception:</p>
  
  <pre><code>"&lt;Message: title&gt;" needs to have a value for field "id" before this many-to-many relationship can be used.
  </code></pre>
  
  <p>How to resolve this issue? some friend can help me about this?</p>