Get extra field in model summing all records from lookup document - mongodb

Question

Having this model:
const matchSchema = mongoose.Schema({
location: {type: mongoose.Types.ObjectId, ref: 'Location'},
datetime: Date,
teamAName: String,
teamBName: String,
teamA: [{type: mongoose.Types.ObjectId, ref: 'Player'}],
teamB: [{type: mongoose.Types.ObjectId, ref: 'Player'}],
teamAScore: {type: Number, default: 0},
teamBScore: {type: Number, default: 0},
pichichi: [{type: mongoose.Types.ObjectId, ref: 'Player'}],
mvp: {type: mongoose.Types.ObjectId, ref: 'Player', default:null},
});
"teamA" and "teamB" are lists containing the "_id" of every player. When retrieving a player, I want to retrieve the number of matches that he/she have played. How can I do that? Below my query just retrieving fields from model "Player"
class PlayerController {
getAll(req, res) {
Player.find()
.sort('firstname')
.exec(function(err, players) {
res.send(players);
});
}
}
So, instead of just having this list:
[
{
_id: new ObjectId("6232395d08663294b412d6a1"),
firstname: 'Sam',
lastname: 'Credington',
__v: 0
},
{
_id: new ObjectId("622479f39be8118a52af70e5"),
firstname: 'Santi',
lastname: 'Futsal',
__v: 0
},
{
_id: new ObjectId("6232399608663294b412d6b9"),
firstname: 'Tom',
lastname: 'Hendry',
__v: 0
}
]
I would like to have the amount of matches that every player played:
[
{
_id: new ObjectId("6232395d08663294b412d6a1"),
firstname: 'Sam',
lastname: 'Credington',
matches:6,
__v: 0
},
{
_id: new ObjectId("622479f39be8118a52af70e5"),
firstname: 'Santi',
lastname: 'Futsal',
matches:8,
__v: 0
},
{
_id: new ObjectId("6232399608663294b412d6b9"),
firstname: 'Tom',
lastname: 'Hendry',
matches: 2,
__v: 0
}
]

Answer 1

Here's one way you could do it.
db.players.aggregate([
{
"$lookup": {
"from": "matches",
"let": { "myId": "$_id" },
"pipeline": [
{
"$match": {
"$expr": {
"$in": [ "$$myId", { "$setUnion": [ "$teamA", "$teamB" ] } ]
}
}
},
{ "$count": "numMatches" }
],
"as": "matchCount"
}
},
{
"$set": {
"matches": {
"$ifNull": [ { "$first": "$matchCount.numMatches" }, 0 ]
}
}
},
{ "$unset": "matchCount" }
])
Try it on mongoplayground.net.