Save does not correctly $set subdocuments for timestamp updates

[Vogel612]

What is the current behavior?

When updating a subdocument with javascript and save(), the behaviour is different from dispatching an equivalent $set operation.
Notably this is relevant for timestamps on subdocuments.

If the current behavior is a bug, please provide the steps to reproduce.

The following test case demonstrates the issue observed.

import mongoose from "mongoose";
import assert from "assert";

describe("Test Repro for $set vs. save issues", () => {
    const childSchema = new mongoose.Schema({
        name: String
    }, { timestamps: { createdAt: "created", updatedAt: false }, _id: false });
    const testSchema = new mongoose.Schema({ child: childSchema, age: Number});
    before(async () => {
        await mongoose.connect('mongodb://localhost:27017/test');
    })
    beforeEach(async () => {
        await mongoose.connection.dropDatabase();
    });
    after(async () => {
        await mongoose.disconnect();
    });
    it("Mongoose $set behaves as expected", async () => {
        const Test = mongoose.model('Test', testSchema);

        await Test.create({ age: 10 });
        await Test.findOneAndUpdate({ child: { $exists: false } }, { $set: { child: { name: "Update Creation" }}});

        const updatedParent: any = await Test.findOne({ age: 10 });
        assert(typeof updatedParent.child.created !== "undefined");
    });
    it("Mongoose save behaves as expected", async () => {
        const Test = mongoose.model('Test', testSchema);
        await Test.create({ age: 10 });

        const parentToChange: any = await Test.findOne({ child: { $exists: false }});
        parentToChange.child = { name: "Update Creation" };
        await parentToChange.save();
        
        const updatedParent: any = await Test.findOne({ age: 10 });
        assert(typeof updatedParent.child.created !== "undefined");
    });
});

What is the expected behavior?

Both test cases should pass, yet only the explicit $set behaves as expected.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

I observed the behaviour after updating to version 6.2.9 from previously 5.11.2. In the 5.x version, this worked as expected.

[IslandRhythms]

const mongoose = require('mongoose');

const assert = require('assert');

const childSchema = new mongoose.Schema({
    name: String
}, { timestamps: {createdAt: "created", updatedAt: false}, _id: false});

const testSchema = new mongoose.Schema({ child: childSchema, age: Number});

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

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

    await Test.create({ age: 10 });
    await Test.findOneAndUpdate({ child: { $exists: false } }, { $set: { child: { name: "Update Creation" }}});

    let updatedParent = await Test.findOne({ age: 10 });
    assert(typeof updatedParent.child.created !== "undefined");

    await Test.create({ age: 12 });

    const parentToChange = await Test.findOne({ child: { $exists: false }});
    parentToChange.child = { name: "Update Creation" };
    await parentToChange.save();
    
    updatedParent = await Test.findOne({ age: 12 });
    assert(typeof updatedParent.child.created !== "undefined"); // fails
}

run();

[vkarpov15]

Confirmed. It looks like the cause of this issue is that, because subdocument isNew is the same as parent isNew after #7048, schema timestamps aren't working correctly for subdocuments. We're working on a fix and should have a fix released within the next couple of days.