条件：如果comments里面的所有username，只要有一条等于xxx(比如：Faith)的，就返回整条记录

{
    "_id": ObjectId("5780913bc3666e25282ffdb0"),
    "description": "xx",
    "readed": 2,
    "type": "1",
    "content": "<p>xx</p>",
    "list_img": "xx",
    "comments": [
        {
            "username": "Faith",
            "content": "bbb",
            "avatar": "/static/images/steam_avatar.jpg",
            "sendtime": "2016-07-09 05:53:04"
        },
       {
            "username": "Faith",
            "content": "ccc",
            "avatar": "/static/images/steam_avatar.jpg",
            "sendtime": "2016-07-09 05:53:04"
        },
       {
            "username": "123",
            "content": "zzz",
            "avatar": "/static/images/steam_avatar.jpg",
            "sendtime": "2016-07-09 05:53:04"
        }
    ],
    "author": "admin",
    "sendtime": "2016-07-09 05:52:59",
    "title": "xx"
}