Lookup based on match result - node.js

Question

I have collection name Services:
[
{
"_id": "61dad1d21aa077c61b7bc2aa",
"name": "HomeMaintenance",
"subServices": [
"61dacb86cb94917c1edcea8f",
"61dad5812881410ba441c401"
],
},
{
"_id": "61dad60b2881410ba441c40e",
"name": "HomeMaintenance",
"subServices": [],
}
]
in another hand I have a subServices Collection like this :
[
{
"_id": "61dacb86cb94917c1edcea8f",
"name": "something",
"title": "something else",
"imageUrl": "",
"__v": 0,
"service": "61dad1d21aa077c61b7bc2aa"
},
{
"_id": "61dad5812881410ba441c401",
"name": "Plumbing",
"title": "Plumbing",
"imageUrl": "",
"__v": 0,
"service": "61dad1d21aa077c61b7bc2aa"
}
]
I came up with a solution with two queries like this
const requestedService = (serviceId)=>{
return servicesModel.findById(id);
};
const ids= requestedService.subServices
const subServicesList = (ids) => {
return subServicesModel.find({
_id: {
$in: ids,
},
});
};
which works perfectly fine, I was wondering is there any way to do these queries with one aggregation pipeline with lookup stage, first find the main services from service collection and then from subServices collection find that subServices of service
something like this
const result = await servicesModel.aggregate([
{
$match: { _id: ObjectId(id) },
},
{
$lookup: {
from: "sub_services",
let: { pid: "$_id" },
pipeline: [
{
$match: {
$expr: {
$in: ["$$pid" //>> id in sub_services modal , //>> "array which we get from match" ],
},
},
},
],
as: "subServices",
},
},
]);

Answer 1

The let is used for declaring the variable from the left document.
Specifies variables to use in the pipeline stages. Use the variable expressions to access the fields from the joined collection's documents that are input to the pipeline.
db.services.aggregate([
{
$match: {
_id: ObjectId(id)
},
},
{
$lookup: {
from: "sub_services",
let: {
subServices: "$subServices"
},
pipeline: [
{
$match: {
$expr: {
$in: [
"$_id",
"$$subServices"
]
},
},
},
],
as: "subServices",
},
},
])
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