Populate one collection with lookup in mongoose - node.js

Question

I'm trying to populate a collection with lookup. The thing is I need to join three collections at once using mongoose in express js.
I have three collections namely, users, skills, userskills.
User and UserSkills are connected. Skills and UserSkills are connected. But not User and Skills.
My model is like
users
{
_id: 5ec6d940b98e8f2c3cea5f22,
name: "test one",
email: "test#example.com"
}
skills
{
_id: 5ec786b21cea7d8c8c186a54,
title: "java"
}
user-skills
{
_id: 5ec7879c1cea7d8c8c186a56,
skillId: "5ec786b21cea7d8c8c186a54",
userId: "5ec6d940b98e8f2c3cea5f22",
level: "good"
}
I tried
user = await User.aggregate([{
$lookup: {
from: "userskills",
localField: "_id",
foreignField: "userId",
as: "userskills"
}
}, {
$unwind: {
path: "$userskills",
preserveNullAndEmptyArrays: true
}
}, {
$lookup: {
from: "skills",
localField: "userskills.skillId",
foreignField: "_id",
as: "userskills.skill",
}
}, {
$group: {
_id : "$_id",
name: { $first: "$name" },
skill: { $push: "$skills" }
}
}, {
$project: {
_id: 1,
name: 1,
skills: {
$filter: { input: "$skills", as: "a", cond: { $ifNull: ["$$a._id", false] } }
}
}
}]);
Required result:
{
"users" : [
{
_id: "5ec6d940b98e8f2c3cea5f22"
name: "test one",
email: "testone#example.com",
"skills" : [
{
_id: "5ec7879c1cea7d8c8c186a56",
level: "good",
"skill" : {
_id: "5ec786b21cea7d8c8c186a54",
title: "java"
}
}
]
},
{
_id: "5ec6d940b98e8f2c3cea5f23"
name: "test two",
email: "testtwo#example.com",
"skills" : [
{
_id: "5ec7879c1cea7d8c8c186a57",
level: "good",
"skill" : {
_id: "5ec786b21cea7d8c8c186a55",
title: "php"
}
}
]
}
]
}
when using
user = await User.find().populate('Skills").exec()
The result is coming like
{
"users" : [
{
_id: "5ec6d940b98e8f2c3cea5f22"
name: "test one",
email: "testone#example.com",
"skills" : [
{
_id: "5ec7879c1cea7d8c8c186a56",
level: "good",
skillId: "5ec786b21cea7d8c8c186a55"
}
]
}
]
}
The problem is I need the skill name should also be fetched. Please help me to solve this issue. I'm writing a backend API in nodejs and mongodb.

Answer 1

You can embed that second $lookup as part of custom pipeline:
await User.aggregate([
{
$lookup: {
from: "userskills",
let: { user_id: "$_id" },
pipeline: [
{ $match: { $expr: { $eq: [ "$$user_id", "$userId" ] } } },
{
$lookup: {
from: "skills",
localField: "skillId",
foreignField: "_id",
as: "skill"
}
},
{ $unwind: "$skill" }
],
as: "skills"
}
}
])
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