ExpressJS / Mongoose双向模型通信

Question

Say I have a User model and a Book model in my express app which both refer to each other - how can I use mongoose middleware to keep both models synced when saving either model?

I think it would be nice if I could use the following structure, so that when the name of a Book is modified it's also updated on its author, the User , and vice versa.

The problem then of course is that the User model is referencing the Book model and the Book model is referencing the User model - a circular reference. Is there any way to circumvent this in a neat and conventional way?

user.model.js

const { mongoose } = require('../db/mongoose');
const Schema = mongoose.Schema;

const Book = require('./book.model.js'); // <--- REFERENCING BOOK

const UserSchema = new Schema({
    name: String,
    books: [
        {
            _id: Schema.Types.ObjectId,
            title: String,
        },
    ],
});

UserSchema.pre('save', function() {
    if (this.isModified('name')) {
        Book.findAndUpdate({
            'author._id': this._id,
        }, {
            // update name of book author
            'author.name': this.name,
        });
    }
});

book.model.js

const { mongoose } = require('../db/mongoose');
const Schema = mongoose.Schema;

const User = require('./user.model.js'); // <--- REFERENCING USER

BookSchema = new Schema({
    title: String,
    author: {
        _id: Schema.Types.ObjectId,
        name: String,
    },
});

BookSchema.pre('save', function() {
    if (this.isModified('title')) {
        User.findByIdAndUpdate(this.author._id, {
            // update the book title
            ...
        });
    }
});

Answer 1

My suggestion would be to actually create an association-collection UserBook which you'll reference from both User and Book collections.

UserBook will contain two fields, one for the id of the book and the other for the id of the user .

As for the updates: you'll start by updating the association-collection and then propagate from it to both collections. Say you want, for example, to update a book, what you'll actually do is update the UserBook collection which you'll design in a why to launch updates, something like this:

Answer 2

The documentation for middleware/hooks states that:

Pre and post save() hooks are not executed on update() , findOneAndUpdate() , etc.

Therefore, a call to book.save(); should not trigger the save-middleware of it's user .