Mongodb aggregate lookup for many collections to one nested output - mongodb

Question

I've 3 Collections:
School
{ "id" : { "$numberLong" : "100000" },
"name" : "School1" }
Faculty
{ "id" : { "$numberLong" : "100000" },
"schoolId" : { "$numberLong" : "100000" },
"name" : "Faculty1" }
Subject
{ "id" : { "$numberLong" : "100000" },
"name" : "Subject1" }
Assume there are many of these in each collection. I want to be able to serve an endpoint that takes in an ID and returns the full 3 layered heirarchy (School->Faculty->Subject). How would I return all this data.
Something like:
{
id: 1,
name: "school1",
faculties: [{
id:1000,
name: "faculty1",
subjects: [
{id: 1, name: "sub1"},
{id: 2, name: "sub2"},
{id: 3, name: "sub3"}
]
}]
}

Answer 1

Ok after ages i actually got the solution, which is a lot simpler than the rabbit hole i'd gone down.
{ $match: {id: 100001}},
{ $lookup:
{
from: 'faculties',
localField: 'id',
foreignField: 'schoolId',
as: 'faculties',
}
},
{ $unwind: {
path: "$faculties",
preserveNullAndEmptyArrays: true
}
},
{ $lookup:
{
from: 'subjects',
localField: 'faculties.id',
foreignField: 'facultyId',
as: 'faculties.subjects',
}
}
Which returns the exact output i wanted. The key is the final lookup returning as: 'faculties.subjects' which puts subjects inside faculties which is the first child of schools.
If you need further nesting, you just need to go as: faculties.subjects.students.names for instance each time you get deeper

Answer 2

db.School.aggregate(
// Pipeline
[
// Stage 1
{
$lookup: // Equality Match
{
from: "Faculty",
localField: "id",
foreignField: "schoolId",
as: "faculties"
}
},
// Stage 2
{
$unwind: {
path: "$faculties",
preserveNullAndEmptyArrays: false // optional
}
},
// Stage 3
{
$lookup: // Equality Match
{
from: "Subject",
localField: "faculties.id",
foreignField: "facultyId",
as: "faculties.subjects"
}
},
// Stage 4
{
$group: {
_id: {
id: '$id',
name: '$name'
},
faculties: {
$addToSet: '$faculties'
}
}
},
// Stage 5
{
$project: {
id: '$_id.id',
name: '$_id.name',
faculties: 1
}
},
]
);