Saving a document that was fetched with .select() causes not selected array to be saved empty

[Bene-Graham]

Prerequisites

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

Mongoose version

6.5.2

Node.js version

16.x

MongoDB server version

5.0.6

Description

This seems to have been changed from version 6.2.2 and 6.2.3. Still an issue in 6.6.0.

const dividerSchema = new mongoose.Schema({
    dividerKey: { type: String, default: "" }
}, { _id: false });

const dataSchema = new mongoose.Schema({
    sectionA: {
        include: { type: Boolean, default: true },
        dividers: [dividerSchema]
    },
    sectionB: {
        include: { type: Boolean, default: true },
        dividers: [dividerSchema],
    }
});

For the given schema if we try to retrieve a document

DataModel.findOne({ "_id": createdDocument._id }).select("sectionA.include sectionA.carrierId sectionB").exec();

and then imminently save it the following update query is created

data_xes.updateOne({ _id: new ObjectId("631ba9fbadbfff8b4ab1f888") }, { '$set': { 'sectionA.dividers': [] } }, { session: null })

This is weird since I never selected or have touch that value.

Steps to Reproduce

import * as mongoose from "mongoose";

const connectToMongo = async () => {
    mongoose.set("debug", true);
    await mongoose.connect("", {
        autoIndex: false,
        autoCreate: false
    });
}

const dividerSchema = new mongoose.Schema({
    dividerKey: { type: String, default: "" }
}, { _id: false });

const dataSchema = new mongoose.Schema({
    sectionA: {
        include: { type: Boolean, default: true },
        dividers: [dividerSchema]
    },
    sectionB: {
        include: { type: Boolean, default: true },
        dividers: [dividerSchema],
    }
});

export const DataModel = mongoose.model("data_x", dataSchema);

const doWork = async () => {
    const createdDocument = await DataModel.create({
        sectionA: {
            include: true,
            dividers: [
                {
                    dividerKey: "SOME_DIVIDER_KEY"
                },
                {
                    dividerKey: "SOME_DIVIDER_KEY"
                }
            ]
        },
        sectionB: {
            include: true,
            dividers: [
                {
                    dividerKey: "SOME_DIVIDER_KEY"
                },
                {
                    dividerKey: "SOME_DIVIDER_KEY"
                }
            ]
        }
    });

    const doc = await DataModel.findOne({ "_id": createdDocument._id }).select("sectionA.include sectionA.carrierId sectionB").exec();

    await doc.save();
};

connectToMongo()
.then(async () => await doWork())
.then(async () => {
    await mongoose.disconnect();
    process.exit(0);
})
.catch((err) => {
    console.error(err);
    process.exit(-1);
});

Expected Behavior

I would expect non selected sub array not to update.

[Bene-Graham]

It seems like the non selected array are being defaulted to empty arrays.

const doc = await Test.findOne({ "_id": createdDocument._id }).select("sectionA.include sectionA.carrierId sectionB").exec();

// 6.2.2 undefined
// 6.2.3 []
console.log(doc.sectionA.dividers);