Assuming that I have the following schemes:

Forum.js

var mongoose = require('mongoose')
const User= require('../user/UserModel');
const Schema=mongoose.Schema;



const ForumSchema = new mongoose.Schema({

forumName: {
    type: String
},
forumDescription: {
    type: String
},
ownerID: {
    type: Schema.Types.ObjectId,
    required: true,
    ref: 'User'
}},{
    timestamps: true
});


const Forum = mongoose.model("Forum", ForumSchema);
module.exports = Forum;

User.js

var mongoose = require('mongoose')
const bcrypt = require('bcrypt')

const UserSchema = new mongoose.Schema({
    id: Number,
    userID: {
        type: String,
        unique: true
    },
    userName: String,
    password: String,
    isAdministrator: {
        type: Boolean,
        default: false
    }
}, {
    timestamps: true
});


  
const User = mongoose.model("User", UserSchema);
module.exports = User;

the function for deleting a user:

function deleteByUserId(req, res, next) {

    let userIDD = req.body.userID;
    User.deleteOne({
        userID: userIDD
    }, function(err, result) {
        if (err) {
            console.log("Fehler bei Suche: " + err)
        } else {
            console.log("Alles gut gelaufen.");
            res.send(result)
        }
    })
}

the mongoose-object:

{
    "_id": "61886d45235d4829c7e46a3c",
    "userID": "admin",
    "userName": "Default Administrator Account",
    "password": "$2b$10$20QgJgZWXyFOdJL94C1EVOojBBmzKH7sp/6SADG8z0rwAQ6QbSXc.",
    "isAdministrator": true,
    "createdAt": "2021-11-08T00:20:21.422Z",
    "updatedAt": "2021-11-08T00:20:21.422Z",
    "__v": 0
  }

The OwnerID in the forum schema is a stored user object. The user object that created the forum. Required=true is because a forum must be created by a user. How can I delete the forum object after deleting the user ?