{
 "_id" : ObjectId("59e05fa63e0f5015dcaeadbe"),
 "name" : "santu",
 "mes": "Test Message 1",
 "time" : ISODate("2017-10-13T06:39:34.273Z"),
 "replay" : [
 {
 "time" : ISODate("2017-10-13T06:39:55.365Z"),
 "_id" : ObjectId("59e05fbb3e0f5015dcaeadbf"),
 "replayer" : "santu",
 "replaymes": "Test Message Reply 1"
 }
 ],
 "__v" : 0
 }

The data structure is like this. I want to get the number of embedded documents or all the contents under’ replay’

db.mesModel.aggregate([
 {$project: {num: {$size: '$replay'}}}
 ])

The result of such query language printing is

[ { _id: 59e05fa63e0f5015dcaeadbe, replay: 1 },
 { _id: 59e063d85f465906606add9a, replay: 1 },
 { _id: 59e063da5f465906606add9b, replay: 1 },
 { _id: 59e063dd5f465906606add9c, replay: 1 } ]

The number of child data of all my parent data is printed out

The Chinese documents are not very complete. . . Looking at English is a little stressful.

The specific realization goal is to obtain all “replaymes” in the above data.
I wonder if there is an api that can be obtained directly.
My own implementation is to find the number of embedded data, traverse

db.mesModel.findOne({mes:'xxx'},(err,doc)=>{
 var len = doc.replay.length
 for(let i=0;  i<len;  i++){
 The console.log (doc.replay [I]. replaymes)//has been printed out.
 }
 })

Accurate multi-criteria query of embedded documents, the realization goal is to find eligible replays under known replays by replays and time.

db.mesModel.aggregate(
 {"$project":{"replay":"$replay"}},
 {"$unwind":"$replay"},
 {"$match":{"replay.replaymes":'xxx','replay.time':'xxx'}}
 ,function(err,doc){
 console.log(doc)
 })

If time is used as the condition, it cannot be matched, but if replay.replayer is used as the condition, it can be matched.