how to do aggregation in nodejs - node.js

Question

I have to fetch a record which have the name field that I give and a subjectId field of another collection for which I have the subjectname with me -- e.g., I would like to fetch a student record which have name lora and subjectname as physics (but the problem is I have subjectname with me but the collection student have the field id of that subject which is been already saved to some other collection say subjects --with _id ---subject _id act as foreign field in student record, I have student name and subject name with me, how can I access the student with corresponding fields?

Answer 1

You have two options on how to do this in Mongo, with option number 1 being the one I personally recommend as it's going to be more efficient.
Split this into two queries:
let subject = await subjectCollection.findOne({name: subjectname});
// add validation that subject exists in case needed.
let doc = await firstCollection.findOne({name: name, subjectId: subject._id})
// you can use `find` instead if multiple documents can be matched.
Use $lookup
firstCollection.aggregate([
{
$match: {
name: name
}
},
{
$lookup: {
from: "subject_collection",
let: {subjectName: subjectName},
pipeline: [
{
$match: {
$expr: {
$eq: ["$$subjectName", "$name"]
}
}
}
],
as: "subjects"
}
},
{
$match: {
"subjects.0": {$exists: true}
}
}
])