mongodb查询问题？

应该如何查询某一条记录中“ comments”中的某条呢？为什么通过“_id”查询不到呢？

{
  "_id": "56fa1c5acb169bd213e485de",
  "title": "ReactJS学习笔记",
  "classify": "js",
  "desc": "React是一个JavaScript库文件",
  "author": "root",
  "body": "",
  "hidden": false,
  "meta": {
    "votes": 0,
    "favs": 0
  },
  "date": "2016-03-30T08:20:36.866Z",
  "comments": [
    {
      "body": "aaaa",
      "date": "2016-03-30T08:43:50.401Z",
      "guest": "A",
      "email": "xxxxxx@qq.com",
      "browser": "chrome",
      "_id": "56fb91c6c4b8801709aec38a"
    },
    {
      "_id": "56fb93ddc4b8801709aec38b",
      "browser": "chrome",
      "email": "xxxxxx@icloud.com",
      "guest": "tester",
      "date": "2016-03-30T08:52:45.445Z",
      "body": "bbbbb"
    }
  ],
  "__v": 0
}