MongoDB findOne() returning empty subdocuments array

Question

I am using Mongoose and MongoDB v. 6.4.1. I have defined a document collection with embedded subdocuments using the following Mongoose schemas:

import mongoose, { Collection } from 'mongoose';
const connectStr = 'mongodb://localhost/appdb';
mongoose.set('useFindAndModify', false);

//Open connection to database
mongoose.connect(connectStr, {useNewUrlParser: true, useUnifiedTopology: true})
  .then(
    () =>  {console.log(`Connected to ${connectStr}.`)},
    err => {console.error(`Error connecting to ${connectStr}: ${err}`)}
  );

//Define schema that maps to a document in the Users collection in the appdb
//database.
const Schema = mongoose.Schema;

const roundSchema = new Schema({
  date: {type: Date, required: true},
  course: {type: String, required: true},
  type: {type: String, required: true, enum: ['practice','tournament']},
  holes: {type: Number, required: true, min: 1, max: 18},
  strokes: {type: Number, required: true, min: 1, max: 300},
  minutes: {type: Number, required: true, min: 1, max: 240},
  seconds: {type: Number, required: true, min: 0, max: 60},
  SGS: {type: Number, 
        default: function(){return (this.strokes * 60) + (this.minutes * 60) + this.seconds}
       },
  notes: {type: String, required: true}
});

const userSchema = new Schema({
  id: {type: String, required: true}, //unique identifier for user
  password: String, //unencrypted password (for now!)
  displayName: {type: String, required: true}, //Name to be displayed within app
  authStrategy: {type: String, required: true}, //strategy used to authenticate, e.g., github, local
  profileImageUrl: {type: String, required: true}, //link to profile image
  rounds: [roundSchema],
  securityQuestion: {type: String},
  securityAnswer: {type: String, required: function() {return this.securityQuestion ? true: false}}
});

//Convert schema to model
const User = mongoose.model("User",userSchema); 

In an Express.js GET route, I am using the following code to query for a specific document:

try {
    let thisUser = await User.findOne({id: req.params.userId});
    console.log("thisUser: " + JSON.stringify(thisUser));
    if (!thisUser) {
      return res.status(400).send("No user account with specified userId was found in database.");
    } else {
      return res.status(200).json(thisUser.rounds);
    }
  } catch (err) {
    console.log(err);
    return res.status(400).message("Unexpected error occurred when looking up user in database: " + err);
  }

My console.log statement confirms that the above route in fact obtains the desired document, e.g.:

thisUser: {"_id":"5e6704234f3864318caedd12","id":"chundhau@gmail.com","password":"GoCougs20","displayName":"chundhau@gmail.com","authStrategy":"local","profileImageUrl":"https://www.gravatar.com/avatar/4b565c54d37b3f5ad4caa1c129e865b8","securityQuestion":"First pet?","securityAnswer":"Daisy","__v":0,"rounds":[]}

When I look at this same document in MongoDB Compass Community, I can confirm that its rounds subdocument array has several elements:

However, as shown in the console.log output above, rounds is coming back as an empty array. I have confirmed that (a) rounds is in fact an array (using Array.isArray()) and that (b) rounds has no elements (thisUser.rounds.length === 0).

Shouldn't I be able to access all of the subdocuments through thisUser.rounds? What have I done wrong?

Answer 1

I have discovered a solution. I changed:

let thisUser = await User.findOne({id: req.params.userId});

to

let thisUser = await User.findOne({id: req.params.userId}).lean();

Miraculously, thisuser.rounds was no longer empty. It instead contained all of the array elements I could see when I inspected the document in MongoDB Compass Community!

While this solution worked, I do not know why it worked. If anyone could help me understand what's going on here, I'd appreciate it!