Can't save changed Object back to MongoDB Database

Question

what I am trying to do is find an object with the corresponding object Id, push a new Id into an array of that object (which does work)

and then save the object back into the MongoDB database. But the group.save() just returnes an error.

My Code:

    addPlacemarkToGroup: {
        handler: async function (request, h) {
            const loggedInUser = request.auth.credentials;
            const user = await db.userStore.getUserById(loggedInUser._id);

            const placemark = await db.placemarkStore.getPlacemarkById(request.payload.placemarkId);
            const group = await db.groupStore.getGroupById(request.payload.groupId);

            console.log(group);

            group.arrayOfPlacemarkIds.push(placemark._id);

            await group.save();

            console.log(group);

            return h.redirect("/dashboard");
        },
    },

My Schema:

import Mongoose from "mongoose";
import { User } from "./user.js";
import { Placemark } from "./placemark.js";

const { Schema } = Mongoose;

const groupSchema = new Schema({
    title: String,
    userId: {
        type: Schema.Types.ObjectId,
        ref: "User",
    },
    arrayOfPlacemarkIds: [{type: [Schema.Types.ObjectId], ref:"Placemark"}]
});

export const Group = Mongoose.model("Group", groupSchema);

Answer 1

You didn't share user and placemark so I added a dummy collection for them. One problem i saw was the type of arrayOfPlacemarkIds, it was defined as an array of arrays. Other thing i dont know what is your groupStore so I just used a findOne. This is a one-file working example, just place your connection string, run, copy the body printed in the console and hit a post request to http://localhost:3000/add

const express = require("express");
var cors = require('cors')

const app = express();

app.use(express.json());
app.use(cors());

const mongoose = require('mongoose');
mongoose.set('strictQuery', true);

const { Schema } = mongoose;

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

const placemarkSchema = new Schema({
    name: String
});

const groupSchema = new Schema({
    title: String,
    userId: {
        type: Schema.Types.ObjectId,
        ref: "User",
    },
    arrayOfPlacemarkIds: [{ type: Schema.Types.ObjectId, ref: "Placemark" }] // 🟥 was as array: [Schema.Types.ObjectId] 
});

const Group = mongoose.model("Group", groupSchema);
const User = mongoose.model("User", userSchema);
const Placemark = mongoose.model("Placemark", placemarkSchema);

(async function ConectToDBAndPrepareData() {
    await mongoose.connect("mongodb://@0.0.0.0:50000", { dbName: "" });

    await User.deleteMany();
    await Group.deleteMany();
    await Placemark.deleteMany();

    const user = await User.create({ name: "User" });
    const placemark1 = await Placemark.create({ name: "Placemark 1" });
    const placemark2 = await Placemark.create({ name: "Placemark 2" });
    const group = await Group.create({ titke: "Group", userId: user._id, arrayOfPlacemarkIds: placemark1 });

    const body = {
        userId: user._id.toString(),
        placemarkId: placemark2._id.toString(),
        groupId: group._id.toString()
    }

    console.log(JSON.stringify(body, null, 2)); //🟥 payload to send a POST request to http://localhost:3000/add
})();


app.post("/add", async function (request, h) {
    const placemark = await Placemark.findOne({ _id: request.body.placemarkId });

    if (!placemark) {
        return h.json("placemark not found");
    }

    const group = await Group.findOne({ _id: request.body.groupId });

    if (!group) {
        return h.json("group not found");
    }

    if (group.arrayOfPlacemarkIds.some(id => id.equals(placemark._id))) {
        return h.json("placemark already added");
    }

    group.arrayOfPlacemarkIds.push(placemark._id);

    await group.save();

    return h.json("OK!");
})

const PORT = 3000;
app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});