Mongoose | Middleware | Rollback operations performed by pre/post hooks when an error is thrown

Question

Considering the code below, how can a transaction be implemented in order to ensure that someParentDocument doesn't get deleted and any operations performed inside the hooks are rolledback, when an error is thrown in any of the hooks?

const parentSchema = new mongoose.Schema({
    name: String,
    children: [{ type: mongoose.Schema.Types.ObjectId, ref: "Child" }],
});

const childSchema = new mongoose.Schema({
    name: String,
    parent: { type: mongoose.Schema.Types.ObjectId, ref: "Parent" },
});

parentSchema.pre("remove", async function(next){
    // Add and remove documents to Parent and Child...
    //...
    next();
});

parentSchema.post("remove", async function(parent){
    throw new Error("Exception!");
});


// (...)
try {
    await someParentDocument.delete();  // <-- will trigger the hooks above
} catch {}

Answer 1

TLDR; Mongoose middleware was not designed for this.

This method of inserting transactions is actually patching the middleware functionality, and you are essentially creating an api completely separate from the mongoose middleware.

What would be better is inverting the logic for your remove query in a separate function.

Simple & Intended Solution

Allow a transaction handling method to do its magic, and create a separate remove method for your parent model. Mongoose wraps mongodb.ClientSession.prototype.withTransaction with mongoose.Connection.prototype.transaction and we don't even have to instantiate or manage a session! Look at the different between the length of this and that below. And you save the mental headache of remembering the internals of that middleware at the cost of one separate function.

const parentSchema = new mongoose.Schema({
    name: String,
    children: [{ type: mongoose.Schema.Types.ObjectId, ref: "Child" }],
});

const childSchema = new mongoose.Schema({
    name: String,
    parent: { type: mongoose.Schema.Types.ObjectId, ref: "Parent" },
});

// Assume `parent` is a parent document here
async function fullRemoveParent(parent) {
    // The document's connection
    const db = parent.db;

    // This handles everything with the transaction for us, including retries
    // session, commits, aborts, etc.
    await db.transaction(async function (session) {
        // Make sure to associate all actions with the session
        await parent.remove({ session });
        await db
            .model("Child")
            .deleteMany({ _id: { $in: parent.children } })
            .session(session);
    });

    // And done!
}

Small Extension

Another way to make this easy, is to register a middleware which simply inherits a session iff_ the query has one registered. Maybe throw an error if a transaction has not been started.

const parentSchema = new mongoose.Schema({
    name: String,
    children: [{ type: mongoose.Schema.Types.ObjectId, ref: "Child" }],
});

const childSchema = new mongoose.Schema({
    name: String,
    parent: { type: mongoose.Schema.Types.ObjectId, ref: "Parent" },
});

parentSchema.pre("remove", async function () {
    // Look how easy!! Just make sure to pass a transactional 
    // session to the removal
    await this.db
        .model("Child")
        .deleteMany({ _id: { $in: parent.children } })
        .session(this.$session());

    // // If you want to: throw an error/warning if you forgot to add a session
    // // and transaction
    // if(!this.$session() || !this.$session().inTransaction()) {
    //    throw new Error("HEY YOU FORGOT A TRANSACTION.");
    // }
});

// Assume `parent` is a parent document here
async function fullRemoveParent(parent) {
    db.transaction(async function(session) {
        await parent.remove({ session });
    });
}

Risky & Complex Solution

This works, and is totally, horribly complex. Not recommended. Will likely break some day because it relies on intricacies of the mongoose API. I don't know why I coded this, please don't include it in your projects.

import mongoose from "mongoose";
import mongodb from "mongodb";

const parentSchema = new mongoose.Schema({
    name: String,
    children: [{ type: mongoose.Schema.Types.ObjectId, ref: "Child" }],
});

const childSchema = new mongoose.Schema({
    name: String,
    parent: { type: mongoose.Schema.Types.ObjectId, ref: "Parent" },
});

// Choose a transaction timeout
const TRANSACTION_TIMEOUT = 120000; // milliseconds

// No need for next() callback if using an async function.
parentSchema.pre("remove", async function () {
    // `this` refers to the document, not the query
    let session = this.$session();

    // Check if this op is already part of a session, and start one if not.
    if (!session) {
        // `this.db` refers to the documents's connection.
        session = await this.db.startSession();

        // Set the document's associated session.
        this.$session(session);

        // Note if you created the session, so post can clean it up.
        this.$locals.localSession = true;

        //
    }

    // Check if already in transaction.
    if (!session.inTransaction()) {
        await session.startTransaction();

        // Note if you created transaction.
        this.$locals.localTransaction = true;

        // If you want a timeout
        this.$locals.startTime = new Date();
    }

    // Let's assume that we need to remove all parent references in the
    // children. (just add session-associated ops to extend this)
    await this.db
        .model("Child") // Child model of this connection
        .updateMany(
            { _id: { $in: this.children } },
            { $unset: { parent: true } }
        )
        .session(session);
});

parentSchema.post("remove", async function (parent) {
    if (this.$locals.localTransaction) {
        // Here, there may be an error when we commit, so we need to check if it
        // is a 'retryable' error, then retry if so.
        try {
            await this.$session().commitTransaction();
        } catch (err) {
            if (
                err instanceof mongodb.MongoError &&
                err.hasErrorLabel("TransientTransactionError") &&
                new Date() - this.$locals.startTime < TRANSACTION_TIMEOUT
            ) {
                await parent.remove({ session: this.$session() });
            } else {
                throw err;
            }
        }
    }

    if (this.$locals.localSession) {
        await this.$session().endSession();
        this.$session(null);
    }
});

// Specific error handling middleware if its really time to abort (clean up
// the injections)
parentSchema.post("remove", async function (err, doc, next) {
    if (this.$locals.localTransaction) {
        await this.$session().abortTransaction();
    }

    if (this.$locals.localSession) {
        await this.$session().endSession();
        this.$session(null);
    }

    next(err);
});