Remove field from embedded projection document after lookup - mongodb

Question

I have the following mongo aggregate query:
return db.collection('projects').aggregate([
{
$match: {
agents: ObjectId(agent)
}
},
{
$lookup: {
from: "agents",
localField: "agents",
foreignField: "_id",
as: "agents"
}
},
{
$lookup: {
from: "roles",
localField: "roles",
foreignField: "_id",
as: "roles"
}
},
{
$lookup: {
from: "agencies",
localField: "agency",
foreignField: "_id",
as: "agency"
}
},
{
$lookup: {
from: "resources",
localField: "roles.applicants",
foreignField: "_id",
as: "roles.applicants"
}
}
])
It works as it should, embedding the proper documents. However, the "password_hash" field is showing for each applicant. I want to remove that field. If I try to project and set roles.applicants.password_hash: 0 I actually end up getting the entire applicant without the password hash, but the rest of the roles fields are no longer there. So I get something that looks like:
roles: {
applicants: {
name: "Josh"
}
}
That should be
roles: {
title: "Super Hero",
applicants: {
name: "Josh"
}
}

Answer 1

I figured it out. Here is how I did it. First, the projection wasn't the issue with why the roles document was missing fields. It was the lookup.
What I did was changed the roles lookup to use the pipeline method on the nested documents and then did another pipeline on applicants, which first matched the applicants from the resource collection and then handled the projection. It looks like this.
{
$lookup: {
from: "roles",
let: { "roles": "$roles" },
pipeline: [
{ $match: { $expr: { $in: [ "$_id", "$$roles" ] } } },
{
$lookup: {
from: "resources",
let: { "applicants": "$applicants" },
pipeline: [
{ $match: { $expr: { $in: [ "$_id", "$$applicants" ] } } },
{
$project: {
first_name: 1
}
}
],
as: "applicants"
}
}
],
as: "roles"
}
}
The entire aggregate looks like this
return db.collection('projects').aggregate([
{
$match: {
agents: ObjectId(agent)
}
},
{
$lookup: {
from: "agents",
localField: "agents",
foreignField: "_id",
as: "agents"
}
},
{
$lookup: {
from: "agencies",
localField: "agency",
foreignField: "_id",
as: "agency"
}
},
{
$lookup: {
from: "roles",
let: { "roles": "$roles" },
pipeline: [
{ $match: { $expr: { $in: [ "$_id", "$$roles" ] } } },
{
$lookup: {
from: "resources",
let: { "applicants": "$applicants" },
pipeline: [
{ $match: { $expr: { $in: [ "$_id", "$$applicants" ] } } },
{
$project: {
first_name: 1
}
}
],
as: "applicants"
}
}
],
as: "roles"
}
}
])