Mongoose not populating related array - node.js

Question

I have followed the mongoose docs for populating related objects, everytime authSessions is empty. I can see in mongo sessions are being populated along with the related userId field. I have tried specifying ”path” and “model” options in the populate function as well but that didn’t work either. Anyone have any ideas?
//Session.js
const SessionSchema = new mongoose.Schema({
_id: Schema.Types.ObjectId,
sessionId: { type: String, index: true },
createdDate: { type: Date, default: Date.now },
revoked: Boolean,
revokedAt: Date,
userId: {type: Schema.Types.ObjectId, ref: 'User', index: true}
})
module.exports = mongoose.models.Session || mongoose.model('Session', SessionSchema);
//User.js
const UserSchema = new mongoose.Schema({
_id: Schema.Types.ObjectId,
username: { type: String, index: true },
email: { type: String, index: true },
password: String,
registrationDate: { type: Date, default: Date.now },
authSessions: [{type: Schema.Types.ObjectId, ref: 'Session'}]
})
module.exports = mongoose.models.User || mongoose.model('User', UserSchema);
const user = await User.findById(session.userId).populate('authSessions').exec();
console.log(user) //authSessions is empty
{
_id: new ObjectId("62b6ea393e042868caa68c7d"),
username: 'asdfasdfasdf',
email: 'testaskldjflk#djlkajdf.com',
password: 'testaskldjflk#djlkajdf.com',
authSessions: [], //empty here always
registrationDate: 2022-06-25T10:58:01.709Z,
__v: 0
} ```

Answer 1

MongoDB, and by extension mongoose, doesn't automatically create relations for you.
If you create a Session document, you need to explicitly add it to a User document if you want populate to work:
// create a new session document and save it
const userSession = new Session(…);
await userSession.save();
// !! update the user document and save it
user.authSessions.push(userSession);
await user.save();