virtual setter called with value undefined on populate

[SinBirb]

Prerequisites

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

Mongoose version

8.1.0

Node.js version

18.18.0

MongoDB server version

7.0.5

Typescript version (if applicable)

No response

Description

Mongoose populates the virtual internally (set() is never called with the populated value), but additionally calls its setter function with the value undefined. This can lead to bad, unexpected side effects.

I'm not sure whether that is expected behavior, but if so, please advise on how we should implement the below use case.

Steps to Reproduce

Example script which outputs a person with userId: null and userType: null.

const mongoose = require('mongoose');
const uri = "mongodb://mongo1:27017/test";

async function run() {
    try {
        await mongoose.connect(uri);
        console.log("connected");

        const Schema = mongoose.Schema;
        const ObjectId = Schema.ObjectId;

        const userSchema = new Schema({
            email: String,
            name: "String",
        });

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

        const user = await User.updateOne({
            email: "admin@example.com",
        }, {
            email: "admin@example.com",
            name: "Admin",
        }, {
            upsert: true,
        });

        const personSchema = new Schema({
            userId: ObjectId,
            userType: String,
        });

        personSchema.virtual('user', {
            ref() {
                return this.userType;
            },
            localField: "userId",
            foreignField: "_id",
            justOne: true,
        })
        .set(function (user) {
            console.log("set user:", user);
            if (user) {
                this.userId = user._id;
                this.userType = toModelName(user);
            } else {
                this.userId = null;
                this.userType = null;
            }
        });

        personSchema.pre("findOne", function () {
            this.populate("user");
        });

        const Person = mongoose.model('Person', personSchema);

        const fetchedUser = await User.findOne({ email: "admin@example.com" })

        const person = new Person({
            userId: fetchedUser._id,
            userType: "User",
        });

        await person.save();

        const fetchedPerson = await Person.findOne({ _id: person._id });

        console.log("person fetched:", fetchedPerson.toJSON({ virtuals: true }));
    } catch (e) {
        console.error(e);
    } finally {
        await mongoose.disconnect();
    }
}

run().catch(console.dir).then(() => console.log("Done"));

Expected Behavior

The populated field (i.e. person) to be populated and the setter not to be called or called with the value that was populated.