mongo 聚合查询 分组，求最大值

数据
聚合查询实在是要了老命了，下面是模拟数据：

 {
    "_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
按照deviceType分类，获得其中每个分类最新的一个版本号

结果举例

 /* 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

 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")
}

db.appPublish.aggregate([
{$group : {_id :{deviceType:"$deviceType,"version:"$version",},creation : {$max : "$creation"}}},
{$project:{_id:1,creation:1}}
]);

楼上的结果

 /* 1 */
{
    "_id" : {
        "deviceType" : "room",
        "version" : "1.1.112"
    },
    "creation" : ISODate("2017-05-05T06:02:50.397Z")
}

/* 2 */
{
    "_id" : {
        "deviceType" : "nurse",
        "version" : "1.1122"
    },
    "creation" : ISODate("2017-05-05T06:02:16.959Z")
}

/* 3 */
{
    "_id" : {
        "deviceType" : "nurse",
        "version" : "1.1。2。2"
    },
    "creation" : ISODate("2017-05-05T06:02:29.447Z")
}

/* 4 */
{
    "_id" : {
        "deviceType" : "bed",
        "version" : "1.1。2。2"
    },
    "creation" : ISODate("2017-05-05T06:02:36.164Z")
}

/* 5 */
{
    "_id" : {
        "deviceType" : "bed",
        "version" : "1.1.112"
    },
    "creation" : ISODate("2017-05-05T06:02:44.750Z")
}

db.appPublish.aggregate({$group:{_id:'$deviceType',version:{$max:'$version'}}})
1._id不能映射到对象上：在映射对象的属性上通过注解指定_id
2.version不能写在group中：如上可以在group中写version_