mongoose aggregate lookup in project - node.js

Question

I currently have 3 different collections.
ColA
{
id: 1,
some_other_data: "fff"
}
ColB
{
id: 1,
colA_id: 1,
spec_id: 5,
data: "test"
}
and
ColC
{
id: 5,
colA_id: 1,
name: "xxx"
}
My current code:
const list = await ColA.aggregate([
{
$lookup: {
from: "ColB",
localField: "_id",
foreignField: "ColA_id",
as: "col_b_list",
},
},
{
$lookup: {
from: "ColC",
localField: "col_b_list.spec_id",
foreignField: "_id",
as: "col_c_list",
},
},
{
$project: {
_id: 1,
list: "$col_b_list",
},
},
]);
return list;
I have following output.
[
{
"_id": "6123858b5b8dcc0e749c9e39",
"list": [
{
"_id": "6123858b5b8dcc0e749c9e43",
"spec_id": "60d33125f81840c010052e03",
"createdAt": "2021-08-23T11:24:59.292Z",
"updatedAt": "2021-08-23T11:24:59.292Z",
"__v": 0
},
{
"_id": "612386317dd1cb0ebcef1862",
"spec_id": "60d33125f81840c010052e03",
"createdAt": "2021-08-23T11:27:45.515Z",
"updatedAt": "2021-08-23T11:27:45.515Z",
"__v": 0
}
]
}
]
What I'm trying to achieve is to get spec_id in ColB, and the id from ColC together in one object.
But what I want is, that in the list object the looked up Collection ColC is included, like:
[
{
"_id": "6123858b5b8dcc0e749c9e39",
"some_other_data": "fff",
"list": [
{
"_id": "6123858b5b8dcc0e749c9e43",
"spec_id": "60d33125f81840c010052e03",
"name": "xxx",
"createdAt": "2021-08-23T11:24:59.292Z",
"updatedAt": "2021-08-23T11:24:59.292Z",
"__v": 0
},
{
"_id": "612386317dd1cb0ebcef1862",
"spec_id": "60d33125f81840c010052e03",
"name": "yyy",
"createdAt": "2021-08-23T11:27:45.515Z",
"updatedAt": "2021-08-23T11:27:45.515Z",
"__v": 0
}
]
}
]
I tried to map it in the project stage, but somehow there where multiple same objects, so I could not get it working properly.
Thanks for the help in advance!

Answer 1

You can use nested $lookup, using lookup with aggregation pipeline,
const list = await ColA.aggregate([
{
$lookup: {
from: "ColB",
let: { id: "$_id" },
pipeline: [
{
$match: {
$expr: { $eq: ["$$id", "$ColA_id"] }
}
},
{
$lookup: {
from: "ColC",
localField: "spec_id",
foreignField: "id",
as: "ColC"
}
},
{
$addFields: {
ColC: { $arrayElemAt: ["$ColC", 0] }
}
},
// { $project: {} }
],
as: "list"
}
}
])