I can't populate data using MongoDB and Node - node.js

Question

these are the two models that I have.
user.js
const mongoose = require("mongoose");
const userSchema = new mongoose.Schema({
email: {
type: String,
},
passwordHashed: {
type: String,
},
role: {
type: String,
},
student: [{ type: mongoose.Schema.Types.ObjectId, ref: "Student" }],
});
const Users = mongoose.model("User", userSchema);
module.exports = Users;
student.js
const mongoose = require("mongoose");
const studentSchema = new mongoose.Schema({
email: {
type: String,
},
role: {
type: String,
},
});
const Students = mongoose.model("Student", studentSchema);
module.exports = Students;
My problem is I want to populate the data to user.js but when I am running this code:
const user = await Users.find().populate('student')
console.log(user)
It just returns me an empty array of the student. How can I fix this problem? Thank you in advance.

Answer 1

For any type population/join I all time use mongodb $lookup.
It's much powerful, flexible and covenant than using population.
According your userSchema, most probably you have store the _id of student document on userSchema's student array.
If I'm correct then you can follow this way:
await Users.aggregate([
{
"$lookup": {
"from": "students",
"localField": "student",
"foreignField": "_id",
"as": "studentList"
}
}
])
Though your student field of userSchema is objectId, then you don't further need to do anything, else you have to convert your localField as mongoose.Types.ObjectId(student)
here is an example: https://mongoplayground.net/p/fuw9Drld9M-