findOneAndUpdate discriminator key does not survive projection

[OoDeLally]

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

What is the current behavior?
When using a discriminator key, all mongoose methods return the discriminator key in addition of the provided projection.
e.g. A {name: 1} projection would still result in a document including the discriminator {name: 'Batman', kind: 'Hero'}.

findOneAndUpdate differs from this behavior, in that it does not include the discriminator key.
e.g. {name: 1} projection results in {name: 'Batman'}.

mongoose 6.1.5

[IslandRhythms]

@OoDeLally Modify the script below to demonstrate your issue. If I had to guess, you were doing the findOneAndUpdate operation on your Main model and not the discriminator.

const mongoose = require('mongoose');

const testSchema = new mongoose.Schema({
    name: String,
});

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

const TestScrim = Test.discriminator('TestScrim', new mongoose.Schema({
    age: Number
}, {discriminatorKey: 'kind'}));

async function run() {
await mongoose.connect('mongodb://localhost:27017');
await mongoose.connection.dropDatabase();

await Test.create({
    name: 'Batman'
});

await TestScrim.create({
    name: 'Nightwing',
    age: 18
});

await TestScrim.create({
    name: 'Batman',
    age: 36
})

console.log(await Test.find({name: 'Batman'}, {name: 1}));

const result = await TestScrim.findOneAndUpdate({age: 36}, {name: 'batman'}, {name: 1, new: true})

console.log(result);
}
run();

[OoDeLally]

Hello @IslandRhythms, thank you for your prompt answer.

I found out that it happens when the discriminator key is part of the schema:

const mongoose = require('mongoose');

const baseSchema = new mongoose.Schema({
    kind: { type: String }, // Comment out to get back to normal behavior.
    name: String,
}, {discriminatorKey: 'kind'});

const BaseModel = mongoose.model('Base', baseSchema);

const childSchema = new mongoose.Schema({
    age: Number
}, { discriminatorKey: 'kind' });

const ChildModel = BaseModel.discriminator('Child', childSchema);

async function run() {
    await mongoose.connect('mongodb://localhost:27017');
    await mongoose.connection.dropDatabase();

    await ChildModel.create({
        name: 'Batman',
        age: 36
    })

    const findResult = await BaseModel.findOne({age: 36}, {name: 1})
    console.log('findResult');
    console.log(findResult);

    const findOneAndUpdateResult = await BaseModel.findOneAndUpdate({age: 36}, {name: 'batman'}, {new: true, projection: { name: 1 }})
    console.log('findOneAndUpdateResult');
    console.log(findOneAndUpdateResult);

    await mongoose.connection.close();
}

run();

What is mongoose's recommendation regarding adding the discriminator to the schema ? I use typescript enums as discriminator values, and therefore it is useful for me to get the validation.

[IslandRhythms]

What is mongoose's recommendation regarding adding the discriminator to the schema?

If you mean defining the discriminator on the schema, it is a feature that will be shipped in the nearby future, possible in mongoose 6.3.
If not, here is the docs regarding discriminators
https://mongoosejs.com/docs/discriminators.html

[vkarpov15]

In #4629 we made it so that Model.find() and Model.findOne() always add the discriminator key to the projection. In 6.3.2, we'll have a fix that will let you do:

const findResult = await BaseModel.findOne({age: 36}, {name: 1, __t: 0})

to opt out of automatically selecting the discriminator key.