Default array not being hydrated when using dotted path projection

[toverux]

Hello,

I think I hit a corner case of the default value hydration, combined with dotted path projection; they don't seem to play well with each other.

This demonstration should sum it up. I have an empty document in database, that I query with findOne(). In the third console.log(), where dotted path notation is used for the projection, array property is no longer being populated with the default value, while object still is (as expected).

const schema = new mongoose.Schema({
    array: {
        type: [{ nestedProperty: String }],
        default: []
    },

    object: {
        type: { nestedProperty: String },
        default: {}
    }
});

const model = mongoose.model('Model', schema);

// OK, result: { array: [], object: {} }
console.log((await model.findOne()));

// OK, result: { array: [], object: {} }
console.log((await model.findOne().select({ 'array': true, 'object': true })));

// KO, result: { object: {} }, expected: { array: [], object: {} }
console.log((await model.findOne().select({ 'array.nestedProperty': true, 'object.nestedProperty': true })));

If I'm not mistaken this should not be the expected behavior.

Note: if the live document has the array property defined, everything works as expected.

---

Mongoose v6.2.1 is being used, but I was able to reproduce the problem as far as v5.3, the oldest version that works with my project without making bigger changes — so this must be an old behaviour and not a recent regression.

Node 16.13.2.

[IslandRhythms]

const mongoose = require('mongoose');

const schema = new mongoose.Schema({
    array: {
        type: [{ nestedProperty: String }],
        default: []
    },

    object: {
        type: { nestedProperty: String },
        default: {}
    }
});

const Test = mongoose.model('Test', schema);

async function run() {

    await mongoose.connect('mongodb://localhost:27017');
    await mongoose.connection.dropDatabase();

    await Test.create({});

// OK, result: { array: [], object: {} }
console.log((await Test.findOne()));

// OK, result: { array: [], object: {} }
console.log((await Test.findOne().select({ 'array': true, 'object': true })));

// KO, result: { object: {} }, expected: { array: [], object: {} }
console.log((await Test.findOne().select({ 'array.nestedProperty': true, 'object.nestedProperty': true })));
}

run();

[toverux]

Hi, thanks for having taken a look.

Remember, I said that there is no problem when the properties are set on the live document — eventually showing that it's not a problem with the projection for example, but rather with the default values.
When using Test.create() in your test case, defaults are being set on the document before it's even created, so everything works as expected because default value hydration doesn't happen with find() and its projection, but before insertion.

To reproduce, in your snippet, just replace this:

await Test.create({});

With:

await mongoose.connection.collection('tests').insertOne({});

On my side, I was able to reproduce the problem again like this.

[IslandRhythms]

const mongoose = require('mongoose');

const schema = new mongoose.Schema({
    array: {
        type: [{ nestedProperty: String }],
        default: []
    },

    object: {
        type: { nestedProperty: String },
        default: {}
    }
});

const Test = mongoose.model('Test', schema);

async function run() {

    await mongoose.connect('mongodb://localhost:27017');
    await mongoose.connection.dropDatabase();

    await mongoose.connection.collection('tests').insertOne({})

// OK, result: { array: [], object: {} }
console.log((await Test.findOne()));

// OK, result: { array: [], object: {} }
console.log((await Test.findOne().select({ 'array': true, 'object': true })));

// KO, result: { object: {} }, expected: { array: [], object: {} }
console.log((await Test.findOne().select({ 'array.nestedProperty': true, 'object.nestedProperty': true })));
}

run();

[toverux]

Thanks @IslandRhythms @vkarpov15!