Mongoose: Problem populating nested array with aggregate - mongodb

Question

I have these models:
const UserSchema = new Schema({
profile: {
type: Schema.Types.ObjectId,
ref: "profiles",
},
});
And this model:
const ProfileSchema = new Schema({
user: {
type: Schema.Types.ObjectId,
ref: "users",
},
education: [
{
institution: {
type: Schema.Types.ObjectId,
ref: "institutions",
},
major: {
type: Schema.Types.ObjectId,
ref: "majors",
},
},
],
date: {
type: Date,
default: Date.now,
},
});
I've been trying to populate the user.profile.education array using aggregate.
Particularly, the fields institution and major.
So the expected result is the array of education to have its education elements populated.
So the expected result should be something like this:
[
// user 1
{
profile: {
education: [
{ institution: "institution_1_data", major: "major_1_data" },
{ institution: "institution_2_data", major: "major_2_data" },
{ institution: "institution_3_data", major: "major_3_data" },
],
},
},
// user 2
{
profile: {
education: [
{ institution: "institution_1_data", major: "major_1_data" },
{ institution: "institution_2_data", major: "major_2_data" },
],
},
},
];
This is the query that I wrote:
const getUsersWithPopulatedMajorAndInstitution = async () => {
const unwind_education_stage = {
$unwind: "$education",
};
const populate_education_stage = {
$lookup: {
from: "majors",
let: { major: "$education.major" },
pipeline: [{ $match: { $expr: { $eq: ["$_id", "$$major"] } } }],
as: "education.major",
},
$lookup: {
from: "institutions",
let: { institution: "$education.institution" },
pipeline: [{ $match: { $expr: { $eq: ["$_id", "$$institution"] } } }],
as: "education.institution",
},
};
const populate_profile_stage = {
$lookup: {
from: "profiles",
let: { profile_id: "$profile" },
pipeline: [
{
$match: {
$expr: { $eq: ["$_id", "$$profile_id"] },
},
},
unwind_education_stage,
populate_education_stage,
{
$project: {
education: "$education",
},
},
],
as: "profile",
},
};
let users = await User.aggregate([populate_profile_stage]);
return users;
};
There are two problems with this query.
PROBLEM 1:
It only populates institution because the institution $lookup stage was added after the major $lookup stage.
This makes no sense to me, as I've been using aggregate for a while and would expect both major and institution to be populated.
PROBLEM 2:
Using $unwind means education field would be unwinded.
So if the education array contains more than 1 education element (like the examples above), three "copies" of the user will be created and the end result is something like this:
[
// user 1
{
profile: {
education: [{ institution: "institution_1_data", major: "major_1_data" }],
},
},
{
profile: {
education: [{ institution: "institution_2_data", major: "major_2_data" }],
},
},
{
profile: {
education: [{ institution: "institution_3_data", major: "major_3_data" }],
},
},
// user 2
{
profile: {
education: [{ institution: "institution_1_data", major: "major_1_data" }],
},
},
{
profile: {
education: [{ institution: "institution_2_data", major: "major_2_data" }],
},
},
];
But, that's not the expected result as I mentioned above.
What should I change/add in the query?

Answer 1

Solved this this way:
const getUsersWithPopulatedMajorAndInstitution = async (
user_name_surname_input_value
) => {
const unwind_education_stage = {
$unwind: "$education",
};
const look_up_institution_stage = {
$lookup: {
from: "institutions",
localField: "education.institution",
foreignField: "_id",
as: "education.institution",
},
};
const look_up_major_stage = {
$lookup: {
from: "majors",
localField: "education.major",
foreignField: "_id",
as: "education.major",
},
};
const populate_stage = {
$lookup: {
from: "profiles",
let: { profile_id: "$profile" },
pipeline: [
{
$match: {
$expr: { $eq: ["$_id", "$$profile_id"] },
},
},
unwind_education_stage,
look_up_major_stage,
look_up_institution_stage,
// This is necessary to regroup education elements after unwinding them
{
$group: {
_id: null,
education: {
$push: "$education",
},
},
},
{
$project: {
education: "$education",
},
},
],
as: "profile",
},
};
let filtered_users = await User.aggregate([
populate_stage,
]);
return filtered_users;
};