Mongoose aggregate pipeline lookup and nested objects - node.js

Question

I would like to share my problem with you.
So I have a 3 entities that needs to be accessed on my query:
Evaluation:
[
{
_id: 1,
questionary: 1,
subject: 1
},
{
_id: 1,
questionary: 1,
subject: 2
},
]
User
[
{
_id: 1
name: "John Doe",
photo: "photo1.jpg"
},
{
_id: 2
name: "Paul Smith",
photo: "photo2.jpg"
},
]
questionary
[
{
_id: 1,
title: "questionary 1",
"date": "2020-02-08T00:00:00.000Z"
},
{
_id: 2,
title: "questionary 2",
"date": "2020-02-09T00:00:00.000Z"
}
]
So my target is getting a data like this: A list of questionaries inside that, a list of evaluations related to a questionary, and inside a evaluation I need a user object. Like this:
[
{
"_id": "1",
"title": "questionary 1",
"evaluations": [
{
"_id": "1",
"date": "2020-04-05T18:53:46.948Z"
"user": {
_id: 1,
"name": "John Doe",
"photo": "photo1.jpg"
}
},
{
"_id": "2",
"date": "2020-04-06T18:53:46.948Z",
"user": {
_id: 1,
"name": "John Doe",
"photo": "photo1.jpg"
}
}
]
}
]
My query is:
return await Questionary.aggregate([{
$lookup: {
from: "evaluation",
localField: "_id",
foreignField: "questionary",
as: "evaluations",
}
},
{
$lookup: {
from: "user",
localField: "evaluations.user",
foreignField: "_id",
as: "user",
}
},
{
$project: {
_id: 1,
title: 1,
status: 1,
evaluations: {
_id: 1,
date: 1,
user: "$user"
}
},
},
]);
And my result is:
[
{
"_id": "1",
"title": "questionary 1",
"evaluations": [
{
"_id": "1",
"date": "2020-04-05T18:53:46.948Z"
"user": [
{
_id: 1,
"name": "John Doe",
"photo": "photo1.jpg"
},
{
_id: 2,
"name": "Paul Smith",
"photo": "photo2.jpg"
}
]
},
{
"_id": "2",
"date": "2020-04-06T18:53:46.948Z",
"user": [
{
_id: 1,
"name": "John Doe",
"photo": "photo1.jpg"
},
{
_id: 2,
"name": "Paul Smith",
"photo": "photo2.jpg"
}
]
}
]
}
]
The users of my evaluations are merging, but this is not that I want, I just want the internal information of the user inside the evaluation.
Any suggestion?

Answer 1

You should be able to use a nested $lookup by using uncorrelated sub-queries as described at https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#join-conditions-and-uncorrelated-sub-queries
In your case that would be:
return await Questionary.aggregate([{
$lookup: {
from: "evaluation",
let: {questId: "$_id"},
pipeline: [{
$match: {
$expr: {
$eq: ["$$questId", "$questionary"]
}
},
}, {
$lookup: {
from: "user",
localField: "subject",
foreignField: "_id",
as: "user",
}
}],
as: "evaluations"
}
}
]);