Facebook Messenger API：发送结构化消息

当关注时这个例子 https://developers.facebook.com/docs/messenger-platform/quickstart.

messageData = {
"attachment": {
  "type": "template",
  "payload": {
    "template_type": "generic",
    "elements": [{
      "title": "First card",
      "subtitle": "Element #1 of an hscroll",
      "image_url": "http://messengerdemo.parseapp.com/img/rift.png",
      "buttons": [{
        "type": "web_url",
        "url": "https://www.messenger.com/",
        "title": "Web url"
      }, {
        "type": "postback",
        "title": "Postback",
        "payload": "Payload for first element in a generic bubble",
      }],
    },{
      "title": "Second card",
      "subtitle": "Element #2 of an hscroll",
      "image_url": "http://messengerdemo.parseapp.com/img/gearvr.png",
      "buttons": [{
        "type": "postback",
        "title": "Postback",
        "payload": "Payload for second element in a generic bubble",
      }],
    }]
  }
}

我收到以下错误：

\"error\":{\"message\":\"(#100) 元素数据不完整：标题和至少一个其他字段（图像 url、副标题或按钮）必须为非空值\",\"类型\":\"OAuthException\",\"代码\":100,\"fbtrace_id\":\"ElbXaF25+0G\"}

我错过了什么吗？在我看来，所有相关字段都存在。

想通了，用红宝石写这个，我需要打电话to_json在发送之前在哈希上。