Mongodb query problem?

  mongodb, question

How should I query one of “comments” in a record? Why can’t you query through “_id”?

{
 "_id": "56fa1c5acb169bd213e485de",
 "title": "reach js learning notes",
 "classify": "js",
 "desc": "React is a JavaScript library file",
 "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
 }

The most direct query:

db.collection.find(
 {
 _id:ObjectId("56fa1c5acb169bd213e485de")
 }
 )

Then obj.comments.body is directly called in the returned object.