Potentially undocumented migration (breaking) change for populating virtuals with no matches, mongoose 5.x returns an empty array, mongoose 6.x returns undefined.

Do you want to request a feature or report a bug?

Possible bug or potentially undocumented breaking change.

What is the current behavior?

Apologies in advance because this seems like something minor. The current behavior is the following - on mongoose 5.x, or version 5.12.14 to be exact, when you are using the Model.populate({ path: '....' }) method, if there are no documents to be found, the query returns an empty array. On Mongoose@latest (6.x+) the same exact query returns undefined instead.

If the current behavior is a bug, please provide the steps to reproduce.

Matches.js

const mongoose = require('mongoose');
const mongooseLeanVirtuals = require('mongoose-lean-virtuals');
const Schema = mongoose.Schema;

const MatchSchema = new Schema(
    {
        _id: { type: Schema.Types.ObjectId, auto: true },
        timestamp: { type: Number, required: true, min: 1577836800000, max: 4102444800000 },
    },
    {
        toObject: {
            virtuals: true
        },
        toJSON: {
            virtuals: true
        }
    }
);

MatchSchema.virtual('predictions', {
    ref: 'predictions',
    localField: '_id',
    foreignField: 'match_id',
    justOne: false,
});

MatchSchema.plugin(mongooseLeanVirtuals);
const Matches = mongoose.model('matches', MatchSchema);

module.exports = Matches;

Predictions.js

const mongoose = require('mongoose');
const mongooseLeanVirtuals = require('mongoose-lean-virtuals');
const Schema = mongoose.Schema;

const PredictionSchema = new Schema(
    {
        _id: { type: Schema.Types.ObjectId, auto: true },
        match_id: { type: Schema.Types.ObjectId, required: true },
    },
    {
        toObject: {
            virtuals: true
        },
        toJSON: {
            virtuals: true
        }
    }
);

PredictionSchema.plugin(mongooseLeanVirtuals);
const Predictions = mongoose.model('predictions', PredictionSchema);

module.exports = Predictions;

Main.js

const mongoose = require('mongoose');
const mongoose_options = {
    autoIndex: true,
    connectTimeoutMS: 10000,
    socketTimeoutMS: 45000,
    family: 4,
    ssl: true
};

const Matches = require('./models/matches');
const Predictions = require('./models/predictions');

(async () => {
    try {
        await mongoose.connect(
            `mongodb+srv://${USER}:${PASS}@${SERVER_ADDR}/${DB_NAME}?retryWrites=true&w=majority`,
            mongoose_options,
        );

        await runTest();

    } catch (error) {
        console.log(error);
    }
})();

const runTest = async () => {
    await Matches.findOneAndUpdate(
        { timestamp: 1577836800000 },
        { timestamp: 1577836800000 },
        { upsert: true }
    );

    const matches = await Matches.find({})
        .populate({ path: 'predictions' })
        .sort({ timestamp: 'desc' })
        .lean({ virtuals: ['predictions'] });

    console.log(matches);
}

What is the expected behavior?

Suppose that there is a match document with a specific mongo _id, but there are no prediction documents to match the populate() query. The code above outputs the following:

// 5.12.14
[
    {
        _id: 6154f33f92736dfa92eda7c9,
        timestamp: 4102444100000,
        predictions: [] // <= this is version 5.x, it returns an empty array.
    },
]

// 6.0.8
[
    {
        _id: new ObjectId("6154f33f92736dfa92eda7c9"),
        timestamp: 4102444100000,
        predictions: undefined // <= this is version 6.x, and it returns undefined instead.
    },
]

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that “latest” is not a version.

Node v16.9.1 Mongoose 5.12.14 and 6.0.8

---

Anyway, my only small request here is to either document this in the migration guide, or if this is not intended behavior and it’s a bug, then I suppose that I am reporting it now.

Thank you for your time!

[vkarpov15]

@brentmjohnson that is correct, lean() without this package will return undefined rather than empty array if there’s no results.

[brentmjohnson]

We ran into this issue as well during our 5 -> 6 migration. In our case we were not using the mongoose-lean-virtuals package and instead statically setting toJSON: { virtuals: true }, toObject: { virtuals: true } at the parent schema level.

The virtual was defined with the default of justOne: false

Can you confirm it is expected behavior that using lean() with populate virtuals (but without the mongoose-lean-virtuals package) will now return undefined rather than an empty array?