Question

First, I’d like to present the case so that everyone can understand what I mean:

https://segmentfault.com/a/1190000004157112

The link above is an ordinary sqlgroup query, but the problem I am facing now is to use mongo for group queries.

Aggregation queries are really killing themselves. Here is the simulation data:
(Please forgive my data, he is bohemian and loves zhi by)

{
 "_id" : ObjectId("590c15751f70bd329f8a972d"),
 "_class" : "com.birdnest.model.AppPublish",
 "user" : "admin",
 "deviceType" : "nurse",
 "version" : "1.1。  2。  2",
 "message": "Van der Sar 3123213213 Van der Sar",
 "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": "Van der Sar 3123213213 Van der Sar",
 "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": "Van 1123 De Sa 3123213213 Van der Sa",
 "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": "Van 1123 De Sa 3123213213 Van der Sa",
 "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": "Van 1123 De Sa 3123213213 Van der Sa",
 "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": "Van 1123 De Sa 3123213213 Van der Sa",
 "creation" : ISODate("2017-05-05T06:02:50.238Z")
 }

The demand is this
According to deviceType classification, the latest version number of each classification is obtained.

The result I want is 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 I have done this, I cannot show the desired results.

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 grammar
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

_idOf course, there is no real mapping_idUp, here_idrefer togroupKey of.
Not sure about yoursversionAndcreationWhat is the relationship?creationLarger documentsversionMust it be relatively new? If this condition holds, we can do this:

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

$firstThat is,groupThen take the first element. And already press in advancecreationArrange the order, so the first is the latest element.