Can only omit discriminator field if `_id` is also selected

[cg-julian-taylor]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

7.4.0

Node.js version

18.6.1

MongoDB server version

4.1.3

Typescript version (if applicable)

No response

Description

When using the mongoose.select() method I can only select to omit discriminator fields if _id is also selected for ommision.

For example:
mongoose.select('-_id -__t') removes the __t field.
mongoose.select('-__t') does not.

This has been attempted using both array and string notation.

this may be related to #13679

Steps to Reproduce

1. call mongoose.select(['-__t'])
2. console.log the result to see that __t is not removed from results.

Expected Behavior

__t discriminator field should be removed via select method.

[cg-julian-taylor]

I don't know if this helps as its not a script as such, but here is an example of what type of code I'm trying to run:

const userSchema = new mongoose.Schema({
  firstName: String
  lastName: String
});

const User = mongoose.model('User', userSchema);

const joeBloggsUser = new User({firstName: 'Joe', lastName: 'Bloggs'});

const users = await User.find().select(['-__t']);

console.log(users[0].__t === undefined);

[IslandRhythms]

Modify the script to demonstrate your issue.

const mongoose = require('mongoose');

const userSchema = new mongoose.Schema({
  firstName: String,
  lastName: String
});

const User = mongoose.model('User', userSchema);

const accountSchema = User.discriminator('account', new mongoose.Schema({ age: Number }, { discriminatorKey: 'kind' }));

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();
  const joeBloggsUser = await User.create({firstName: 'Joe', lastName: 'Bloggs'});

  const users = await User.find().select(['-__t']);

  console.log(users[0].__t === undefined, users[0].__t);
  const test = new accountSchema({ age: 2 });
  console.log(test.__t)
  await test.save();
  const res = await accountSchema.find().select(['-__t']);
  console.log(res[0].__t)
}

run();

[cg-julian-taylor]

Hi there, I've managed to replicate the issue. Please find enclosed a script to replicate the issue:

const mongoose = require('mongoose');

async function run(){
  const baseConfig = {
    discriminationKey: '_type',
    collection: 'users'
  };

  const NamesSchema = new mongoose.Schema({
    firstName:   { type: String },
    lastName:  { type: String },
    test: {type: String}
  });

  const User = mongoose.model('User', new mongoose.Schema({}, baseConfig));

  const NamesModel = User.discriminator('names', NamesSchema);

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

  await mongoose.connection.dropDatabase();

  const joeBloggsUser = await NamesModel.create({firstName: 'Joe', lastName: 'Bloggs', test: '123'});

  const userNameResult = await NamesModel.findOne({})
    .select(['-__v', '-__t', '-test'])
    .lean();

  console.log(userNameResult.__t === undefined, userNameResult.__t, userNameResult.__v, userNameResult.test);
}

run();