mongodb - mongo aggregation query grouping, finding the maximum value

Question

1. Let me present the case first so that everyone can understand what I mean:

/a/1190000004157112

The above link is an ordinary sqlgroup query, but the problem I am encountering now is using mongo to perform group query

Aggregation query is really killing me. Here is the simulated data:
(Please forgive my data, he is uninhibited and free)

 {
    "_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")
}

The requirements are like this
According to the deviceType classification, get the latest version number of each category

The result I want is like this

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

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

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

The following statement was written by myself, but there are many problems
Although the thing I wrote was done, the desired result could not be displayed

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

1._id cannot be mapped to an object
2.version cannot be written in group

I really don’t understand this syntax
Result

 /* 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")
}

Answer 1

_id当然映射不到真正的_id上,这里的_id指的是group的key。
不清楚你的version和creation是什么关系，creation比较大的文档versionIt must be relatively new? If this condition is true, you can do this:

db.question.aggregate([
    {$sort: {creation: -1}},
    {$group : {_id :"$deviceType", version : {$first : "$version"}}},
    {$project: {deviceType: "$_id", version: "$version"}}
]);

$first即group之后取第一个元素。而事先已经按creationIn order, so the first one is the newest element.