MongoDB multiple/nested aggregations - mongodb

Question

I have these collections:
users
{
_id: "userId1",
// ...
tracks: ["trackId1", "trackId2"],
};
tracks
{
_id: "trackId1",
// ...
creatorId: "userId1",
categoryId: "categoryId1"
}
categories
{
_id: "categoryId1",
// ...
tracks: ["trackId1", "trackId15", "trackId20"],
};
by using the following code, I am able to get a track by its ID and add the creator
tracks.aggregate([
{
$match: { _id: ObjectId(trackId) },
},
{
$lookup: {
let: { userId: { $toObjectId: "$creatorId" } },
from: "users",
pipeline: [{ $match: { $expr: { $eq: ["$_id", "$$userId"] } } }],
as: "creator",
},
},
{ $limit: 1 },
])
.toArray();
Response:
"track": {
"_id": "trackId1",
// ...
"categoryId": "categoryId1",
"creatorId": "userId1",
"creator": {
"_id": "userId1",
// ...
"tracks": [
"trackId5",
"trackId10",
"trackId65"
]
}
}
but what I am struggling with is that I want the creator.tracks to aggregate also returning the tracks by their ID (e.g up to last 5), and also to get the last 5 tracks from the categoryId
expected result:
"track": {
"_id": "trackId1",
// ...
"categoryId": "categoryId1",
"creatorId": "userId1",
"creator": {
"_id": "userId1",
"tracks": [
{
"_id": "trackId5",
// the rest object without the creator
},
{
"_id": "trackId10",
// the rest object without the creator
},
{
"_id": "trackId65",
// the rest object without the creator
},
]
},
// without trackId1 which is the one that is being viewed
"relatedTracks": [
{
"_id": "trackId15",
// the rest object without the creator
},
{
"_id": "trackId20",
// the rest object without the creator
},
]
}
I would appreciate any explanation/help to understand what is the best one to do it and still keep the good performance

Answer 1

Query
start from a track
join with users using the trackId get all the tracks of the creator
(creator-tracks)
join with categories using the categoryId to get all the tracks of the category (related tracks)
remove from related-tracks the tracks of the creator
take the last 5 from both using $slice (creator-tracks and related-tracks)
*i added 2 extra lookups to get all info of the tracks, its empty arrays because i dont have enough data(i have only trackId1), with all the data it will work
PlayMongo
db.tracks.aggregate([
{
"$match": {
"_id": "trackId1"
}
},
{
"$lookup": {
"from": "users",
"localField": "creatorId",
"foreignField": "_id",
"as": "creator-tracks"
}
},
{
"$set": {
"creator-tracks": {
"$arrayElemAt": [
"$creator-tracks.tracks",
0
]
}
}
},
{
"$lookup": {
"from": "categories",
"localField": "categoryId",
"foreignField": "_id",
"as": "related-tracks"
}
},
{
"$set": {
"related-tracks": {
"$arrayElemAt": [
"$related-tracks.tracks",
0
]
}
}
},
{
"$set": {
"related-tracks": {
"$filter": {
"input": "$related-tracks",
"cond": {
"$not": [
{
"$in": [
"$$this",
"$creator-tracks"
]
}
]
}
}
}
}
},
{
"$set": {
"creator-tracks": {
"$slice": [
{
"$filter": {
"input": "$creator-tracks",
"cond": {
"$ne": [
"$$this",
"$_id"
]
}
}
},
-5
]
}
}
},
{
"$set": {
"related-tracks": {
"$slice": [
"$related-tracks",
-5
]
}
}
},
{
"$lookup": {
"from": "tracks",
"localField": "creator-tracks",
"foreignField": "_id",
"as": "creator-tracks-all-info"
}
},
{
"$lookup": {
"from": "tracks",
"localField": "related-tracks",
"foreignField": "_id",
"as": "related-tracks-all-info"
}
}
])