Issues with lookup and match multipe collections - mongodb

Question

Having issues with aggregate and lookup in multiple stages. The issue is that I cannot match by userId In the last lookup. If I omit the { $eq: ['$userId', '$$userId'] } it works and match by the other criteria. But not by the userid.
I've tried added pools as a let and use it as { $eq: ['$userId', '$$pools.userId'] } in the last stage but that doesn't work either. I get an empty coupon array.
I get this with the below query. I think I need to use $unwind in some way? But haven't got that to work yet. Any pointers?
There is three collections total to be joined. First the userModel, it should contain pools and then the pools should contain a users coupons.
{
"userId": "5df344a1372f345308dac12a", // Match this usedId with below userId coming from the coupon
"pools": [
{
"_id": "5e1ebbc6cffd4b042fc081ab",
"eventId": "id999",
"eventStartTime": "some date",
"trackName": "tracky",
"type": "foo bar",
"coupon": []
}
]
},
I need the coupon array to be filled with the correct data (below) which has a matching userId in it.
"coupon": [
{
"eventId": "id999",
"userId": "5df344a1372f345308dac12a", // This userId need to match the above one
"checked": true,
"pool": "a pool",
}
poolProject:
const poolProject = {
eventId: 1,
eventStartTime: 1,
trackName: 1,
type: 1,
};
Userproject:
const userProjection = {
_id: {
$toString: '$_id',
},
paper: 1,
correctBetsLastWeek: 1,
correctBetsTotal: 1,
totalScore: 1,
role: 1,
};
The aggregate query
const result = await userModel.aggregate([
{ $project: userProjection },
{
$match: {
$or: [{ role: 'User' },
{ role: 'SuperUser' }],
},
},
{ $addFields: { userId: { $toString: '$_id' } } },
{
$lookup: {
from: 'pools',
as: 'pools',
let: { eventId: '$eventId' },
pipeline: [
{ $project: poolProject },
{
$match: {
$expr: {
$in: ['$eventId', eventIds],
},
},
},
{
$lookup: {
from: 'coupons',
as: 'coupon',
let: { innerUserId: '$$userId' },
pipeline: [
{
$match: {
$expr: {
$eq: ['$userId', '$$innerUserId'],
},
},
},
],
},
},
],
},
},
]);
Thanks for any input!
Edit:
If i move the second lookup (coupon) so they are in the same "level" it works but i would like to have it inside of the pool. If I add as: 'pools.coupon', in the last lookup it overwrites the lookedup pool data.

Answer 1

When you access fields with the $$ prefix it means they are defined as "special" system variables by Mongo.
We don't know exactly how Mongo the magic happens but you're naming two variables with the same name, which causes a conflict as it seems.
So either remove userId: '$userId' from the first lookup as you're not even using it.
Or rename or second userId: '$userId' a different name like innerUserId: '$userId' to avoid conflicts when you access it.
Just dont forget to change { $eq: ['$userId', '$$userId'] } to { $eq: ['$userId', '$$innerUserId'] } after.
EDIT:
Now that its clear theres no field userId in pools collection just change the variable in the second lookup collection from:
let: { innerUserId: '$userId' } //userId does not exist in pools.
To:
let: { innerUserId: '$$userId' }