Can't join collections with ids in mongodb - node.js

Question

I am trying to get all requests of a user from the request collection based on request status. I am trying to lookup the collection but it doesn't work. Is there any solution to work it out.
Here is my code:
Users.aggregate([
{
$lookup: {
from: 'requests',
let: {userId: '$userId', status: '$status'},
pipeline: [
{
$match: {
$expr: {
$and: [
{ $eq: ['$_id', '$$userId'] },
{ $eq: ['$$status', 1] }
]
},
}
}
],
as: 'requests'
}
}
]).exec()
I think { $eq: ['$_id', '$$userId'] } is not working. I tried using $toObjectId but still same result.
Here is test data for users:
{
"_id": {
"$oid": "5f1c0112ad207a13308a3fea"
},
"createDate": {
"$date": "2020-07-25T09:52:58.678Z"
},
"userRole": 10,
"status": 1,
"fullName": "Test Name",
"email": "test.name#mailinator.com",
"password": "$2b$10$HQN//qFTQKW8tBnf7G0OV.Uta0zNbxd1hPlGVwvLp5CVIf49Y5PNW",
"__v": 0,
"profileImage": "1595957619296.jpg"
}
And test request data:
{
"_id": {
"$oid": "5f2178c578153018ca5d79e8"
},
"request": "This is a demo request.",
"userId": {
"$oid": "5f1c0112ad207a13308a3fea"
},
"createDate": {
"$date": "2020-07-28T18:30:00.000Z"
},
"status": 1
}

Answer 1

There are few fixes in your query,
$match your user document status is 1, if you don't want then your can exclude,
db.users.aggregate([
{
$match: {
status: 1
}
},
$lookup with requests
let userId it is from user collection so add user collection _id so its corrected,
no need to create status variable because we already checked condition for in above
{
$lookup: {
from: "requests",
let: {
userId: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$and: [
$eq check first is requests collection $userId and second is that we have created variable above in let and use $$userId because its reference to main collection users let variables
{
$eq: [
"$userId",
"$$userId"
]
},
second $eq check $status is 1 in requests collection, and you did it in user collection we we have already used in above $match condition
{
$eq: [
"$status",
1
]
}
]
}
}
}
],
as: "requests"
}
}
])
Have divided in parts for explanation purpose you can merge it as they are in sequence,
Working Playground: https://mongoplayground.net/p/THU6HeyqMN4