1. 先把案例奉上，让大家先理解我的意思：

/a/1190000004157112

上面这个链接是一个普通的sqlgroup查询，但是我现在碰到的问题是用mongo进行group查询

聚合查询实在是要了老命了，下面是模拟数据：
（请原谅我的数据，他放荡不羁爱zhi由）

 {
    "_id" : ObjectId("590c15751f70bd329f8a972d"),
    "_class" : "com.birdnest.model.AppPublish",
    "user" : "admin",
    "deviceType" : "nurse",
    "version" : "1.1。2。2",
    "message" : "范德萨312313213213范德萨",
    "creation" : ISODate("2017-05-05T06:02:29.300Z")
}

{
    "_id" : ObjectId("590c15751f70bd329f8a972e"),
    "_class" : "com.birdnest.model.AppPublish",
    "user" : "admin",
    "deviceType" : "nurse",
    "version" : "1.1。2。2",
    "message" : "范德萨312313213213范德萨",
    "creation" : ISODate("2017-05-05T06:02:29.447Z")
}

{
    "_id" : ObjectId("590c157b1f70bd329f8a972f"),
    "_class" : "com.birdnest.model.AppPublish",
    "user" : "admin",
    "deviceType" : "bed",
    "version" : "1.1。2。2",
    "message" : "范1123德萨312313213213范德萨",
    "creation" : ISODate("2017-05-05T06:02:35.731Z")
}

{
    "_id" : ObjectId("590c157c1f70bd329f8a9730"),
    "_class" : "com.birdnest.model.AppPublish",
    "user" : "admin",
    "deviceType" : "bed",
    "version" : "1.1。2。2",
    "message" : "范1123德萨312313213213范德萨",
    "creation" : ISODate("2017-05-05T06:02:36.164Z")
}

{
    "_id" : ObjectId("590c158a1f70bd329f8a9733"),
    "_class" : "com.birdnest.model.AppPublish",
    "user" : "admin",
    "deviceType" : "room",
    "version" : "1.1.112",
    "message" : "范1123德萨312313213213范德萨",
    "creation" : ISODate("2017-05-05T06:02:50.082Z")
}
{
    "_id" : ObjectId("590c158a1f70bd329f8a9734"),
    "_class" : "com.birdnest.model.AppPublish",
    "user" : "admin",
    "deviceType" : "room",
    "version" : "1.1.112",
    "message" : "范1123德萨312313213213范德萨",
    "creation" : ISODate("2017-05-05T06:02:50.238Z")
}

需求是这样的
按照deviceType分类，获得其中每个分类最新的一个版本号

我想要的结果是这样的

 /* 1 */
{
    "deviceType" : "bed",
    "version" :"1.1.112")
}

/* 2 */
{
    "deviceType" : "room",
    "version" :"1.1.112")
}

/* 3 */
{
    "deviceType" : "nurse",
    "version" : "1.1.112")
}

下面这个语句是我自己写的，但是存在很多问题
我写的这个事虽然做了但是想要的结果却展示不出来

 db.appPublish.aggregate(
    {$group : {_id :"$deviceType",creation : {$max : "$creation"}}}
);

1._id不能映射到对象上
2.version不能写在group中

实在有点弄不懂这个语法
结果

 /* 1 */
{
    "_id" : "bed",
    "creation" : ISODate("2017-05-05T06:02:44.750Z")
}

/* 2 */
{
    "_id" : "room",
    "creation" : ISODate("2017-05-05T06:02:50.397Z")
}

/* 3 */
{
    "_id" : "nurse",
    "creation" : ISODate("2017-05-05T06:02:29.447Z")
}