MongoDB aggregation matching ObjectId against string - mongodb

Question

I have the following document which is also available in the mongo playground at:
https://mongoplayground.net/p/zhcoi1BF0Ny
db={
MyCollectionOne: [
{
"firstId": "10",
"secondId": "123456789012345678901234"
},
{
"firstId": "11",
"secondId": "999999999999999999999999"
}
],
MyCollectionTwo: [
{
"_id": ObjectId("123456789012345678901234"),
"otherFieldOne": "Some Data",
"otherFieldTwo": [
{
someNumber: 7
}
]
},
{
"_id": ObjectId("999999999999999999999999"),
"otherFieldOne": "Some Other Data",
"otherFieldTwo": [
{
someNumber: 9
},
{
someNumber: 39
}
]
}
]
}
Given a firstId, I need to determine the correct MyCollectionTwo entry to return. So for example, if I was given a firstId of 11, I would use that to lookup its corresponding secondId (which is "999999999999999999999999"). I would need to convert the secondId value to an ObjectId and then look through the MyCollectionTwo _id fields until I find the matching one.
I gave it a try and am very close but cannot figure out how to correctly do the string->objectId conversion.
db.MyCollectionTwo.aggregate([
{
$lookup: {
from: "MyCollectionOne",
localField: "_id",
foreignField: "secondId",
as: "Temp"
}
},
{
$unwind: "$Temp"
},
{
$match: {
"Temp.firstId": "11"
}
},
{
$project: {
_id: 1,
otherFieldOne: 1,
otherFieldTwo: 1
}
}
]).find()
I am aware there is a let/pipeline which can use a $toObjectId but I can't get that working in the above context.
Any help would be appreciated. Thanks!

Answer 1

Your $lookup with pipeline should be as below:
$lookup: {
from: "MyCollectionOne",
let: {
id: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$eq: [
{
$toObjectId: "$secondId"
},
"$$id"
]
}
}
}
],
as: "Temp"
}
Sample Mongo Playground

Answer 2

You can try this
db.MyCollectionTwo.aggregate([
{
$lookup: {
//searching collection name
from: "MyCollectionOne",
//setting variable [searchId] where your string converted to ObjectId
let: {
"searchId": {
$toObjectId: "$secondId"
}
},
//search query with our [searchId] value
"pipeline": [
//searching [searchId] value equals your field [_id]
{
"$match": {
"$expr": [
{
"_id": "$$searchId"
}
]
}
},
],
as: "Temp"
}
},
{
$unwind: "$Temp"
},
{
$match: {
"Temp.firstId": "11"
}
},
{
$project: {
_id: 1,
otherFieldOne: 1,
otherFieldTwo: 1
}
}
]).find()
https://mongoplayground.net/p/es0j0AiPDCj