.toObject() has different behavior from 6.x to 7.x

[puskin94]

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.x

Node.js version

18.15

MongoDB server version

6.x

Typescript version (if applicable)

5.1.3

Description

The .toObject() method called after the creation of a document has a different behavior when you compare mongoose version 6.x and 7.x

It looks like the deeply nested properties , when set to undefined, and passed to a findByIdAndUpdate in:

• version 6.x are removed
• version 7.x are set to null

removing the toObject() method call make the 2 mongoose versions behave the same way, as displayed in the steps to Reproduce

Steps to Reproduce

const mongoose = require('mongoose');

const mySchema = new mongoose.Schema({
    extended: {
        type:    Object || String,
        default: {keep: true}
    }
});

const myModel = mongoose.model('my', mySchema);

async function main() {
    await mongoose.connect('mongodb://localhost:27017/test-mongoose');

    const myObj = {
        extended: {
            something: {else: true},
            level1:    {level2: {prop: 'something'}}
        }
    };

    let added = (await myModel.create(myObj)).toObject(); // Without the toObject both versions behave the same
    console.log('>>> added:', added.extended); // { something: { else: true }, level1: { level2: { prop: something } } }

    added.extended.level1.level2.prop = undefined;
    console.log('>>> updated', added.extended); // { something: { else: true }, level1: { level2: { prop: undefined } } }

    await myModel.findByIdAndUpdate(added._id, added, {lean: true, new: true, overwrite: true});

    const found = await myModel.findById(added._id);
    console.log('>>> found:', found.extended);
    // 7.x.x { something: { else: true }, level1: { level2: { prop: null } } }
    // 6.x.x { something: { else: true } }
}

main()
    .then(() => process.exit())
    .catch(e => {
        console.error(e);
        process.exit(1);
    });

These are the logs for version 6.x with the toObject() method call:

These are the logs for version 7.x with the toObject() method call:

These are the logs for version 7.x without the toObject() method call:

Expected Behavior

No response

[vkarpov15]

The fundamental issue here is that, in Mongoose 6, findOneAndUpdate() with overwrite: true executes a findOneAndReplace(). We'll add that behavior back to Mongoose 7, but I would recommend you use findOneAndReplace() instead of findOneAndUpdate() with overwrite. We may deprecate the overwrite option in the future.