Pretty $lookup on collection - mongoDB - mongodb

Question

I have two collection:
Competition
{
"_id": "326",
signed_up": [
{"_id": "00001","category": ["First"], "status": true}]
}
and Playing
{
"_id": "6076e504db319b11c077d473",
"competition_id": "326",
"player": {"player_id": "00001","handicap": 6},
"totalScore": 6
}
I want to add playing --> totalScore on competition.signed_up array, based on player_id field:
{
"_id": "326",
signed_up": [
{"_id": "00001","category": ["First"], "status": true, "totalScore": 6]
}
I do not know how to do...

Answer 1

I'm not telling you this is the optimal way, but it seems to work...
Let's start out with the data. I've added one player to the competition, just to make it a little easier to see that things works as expected:
db.competition.insertOne({
"_id": "326",
"signed_up": [{
"_id": "00001",
"category": ["First"],
"status": true
}, {
"_id": "00002",
"category": ["First"],
"status": true
}]
})
db.playing.insertMany([
{
"competition_id": "326",
"player": {
"playing_id": "00001"
},
"totalScore": 6
},
{
"competition_id": "326",
"player": {
"playing_id": "00002"
},
"totalScore": 2
}
]);
Now for the aggregation...
db.competition.aggregate([
// Even though the [documentation](https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#use--lookup-with-an-array) states that unwinding is no longer necessary,
// I'm not sure if that includes arrays of subdocuments or only arrays of primitives. So I've chosen to unwind anyway...
{
$unwind: "$signed_up"
},
// => { "_id": "326", "signed_up": { "_id": "00001", ....} }
// now we have each player in it's own document and can easily lookup the score from playing collection
{
$lookup: {
from: 'playing',
localField: 'signed_up._id',
foreignField: 'player.playing_id',
as: 'player'
}
},
// => { "_id": "326", "signed_up": {...}, "player": [{ competition_id": "326"...}, ..]}
// now we have the matching competition documents as an array on each document.
// But we know there will only be one match and don't really care for the array,
// so we have to do some gymnastics to get the data we want where we want it
{
$project: {
"signed_up": {
$let: {
vars: {
player: { $arrayElemAt: [ "$player", 0 ] }
},
in: {
$mergeObjects: [
"$signed_up",
{ "totalScore": "$$player.totalScore" }
]
}
}
}
}
},
// => { "_id": "326", "signed_up": { "_id": "00001", .... , "totalScore": 6 } }
// Now we're pretty much done, except that we need to group the documents back
// into the original competition documents
{
$group: {
_id: "$_id",
signed_up: {
$push: "$signed_up"
}
}
}
// => { "_id": "326", "signed_up": [ { "_id": "00001", ....}, {"_id": "00002", ...} ] }
// And that completes the pipeline.
]);
I see that you have the id from the competition document also on the playing document, so I suspect that you need an additional check on the lookup to make sure you get the correct match. The way the code I have works, is that if you have more than one competition, you will get all the competitions for a player added to the playing array after the lookup.
If you take a look at the example Specify Multiple Join Conditions with $lookup in the documentation, you see how you can change the $lookup stage to do a more precise match on the target documents by using a pipeline on the target collection. It also shows how you can include a projection in that pipeline to only return the data that you really want.
Edit
Take a look at the following alternative lookup step:
{
$lookup: {
from: 'playing',
let: { playerid: "$signed_up._id", compid: "$_id" },
pipeline: [
{ $match: {
$expr: {
$and: [
{ $eq: ["$player.playing_id","$$playerid" ] },
{ $eq: ["$competition_id", "$$compid" ] }
]
}
}
},
{ $project: {
_id: 0,
"totalScore": 1
}
}
],
as: 'player'
}
}
This stores the players id and competition id from the current document into two variables. Then it uses those two variables in a pipeline run against the other collection. In addition to the $match to select the right player/competition document, it also includes a $project to get rid of the other fields on the playing documents. It will still return an array of one object, but it might save some bytes of memory usage...