Transactions in Mongoose

Transactions are new in MongoDB 4.0 and Mongoose 5.2.0. Transactions let you execute multiple operations in isolation and potentially undo all the operations if one of them fails. This guide will get you started using transactions with Mongoose.

Getting Started with Transactions

If you haven't already, import mongoose:

import mongoose from 'mongoose';

To create a transaction, you first need to create a session using Mongoose#startSession or Connection#startSession().

// Using Mongoose's default connection
const session = await mongoose.startSession();

// Using custom connection
const db = await mongoose.createConnection(mongodbUri).asPromise();
const session = await db.startSession();

In practice, you should use either the session.withTransaction() helper or Mongoose's Connection#transaction() function to run a transaction. The session.withTransaction() helper handles:

• Creating a transaction
• Committing the transaction if it succeeds
• Aborting the transaction if your operation throws
• Retrying in the event of a transient transaction error.

let session = null;
return Customer.createCollection().
  then(() => Customer.startSession()).
  // The `withTransaction()` function's first parameter is a function
  // that returns a promise.
  then(_session => {
    session = _session;
    return session.withTransaction(() => {
      return Customer.create([{ name: 'Test' }], { session: session });
    });
  }).
  then(() => Customer.countDocuments()).
  then(count => assert.strictEqual(count, 1)).
  then(() => session.endSession());

For more information on the ClientSession#withTransaction() function, please see the MongoDB Node.js driver docs.

Mongoose's Connection#transaction() function is a wrapper around withTransaction() that integrates Mongoose change tracking with transactions. For example, suppose you save() a document in a transaction that later fails. The changes in that document are not persisted to MongoDB. The Connection#transaction() function informs Mongoose change tracking that the save() was rolled back, and marks all fields that were changed in the transaction as modified.

const doc = new Person({ name: 'Will Riker' });

await db.transaction(async function setRank(session) {
  doc.name = 'Captain';
  await doc.save({ session });
  doc.isNew; // false

  // Throw an error to abort the transaction
  throw new Error('Oops!');
}, { readPreference: 'primary' }).catch(() => {});

// true, `transaction()` reset the document's state because the
// transaction was aborted.
doc.isNew;

With Mongoose Documents and save()

If you get a Mongoose document from findOne() or find() using a session, the document will keep a reference to the session and use that session for save().

To get/set the session associated with a given document, use doc.$session().

const User = db.model('User', new Schema({ name: String }));

let session = null;
return User.createCollection().
  then(() => db.startSession()).
  then(_session => {
    session = _session;
    return User.create({ name: 'foo' });
  }).
  then(() => {
    session.startTransaction();
    return User.findOne({ name: 'foo' }).session(session);
  }).
  then(user => {
    // Getter/setter for the session associated with this document.
    assert.ok(user.$session());
    user.name = 'bar';
    // By default, `save()` uses the associated session
    return user.save();
  }).
  then(() => User.findOne({ name: 'bar' })).
  // Won't find the doc because `save()` is part of an uncommitted transaction
  then(doc => assert.ok(!doc)).
  then(() => session.commitTransaction()).
  then(() => session.endSession()).
  then(() => User.findOne({ name: 'bar' })).
  then(doc => assert.ok(doc));

With the Aggregation Framework

The Model.aggregate() function also supports transactions. Mongoose aggregations have a session() helper that sets the session option. Below is an example of executing an aggregation within a transaction.

const Event = db.model('Event', new Schema({ createdAt: Date }), 'Event');

let session = null;
return Event.createCollection().
  then(() => db.startSession()).
  then(_session => {
    session = _session;
    session.startTransaction();
    return Event.insertMany([
      { createdAt: new Date('2018-06-01') },
      { createdAt: new Date('2018-06-02') },
      { createdAt: new Date('2017-06-01') },
      { createdAt: new Date('2017-05-31') }
    ], { session: session });
  }).
  then(() => Event.aggregate([
    {
      $group: {
        _id: {
          month: { $month: '$createdAt' },
          year: { $year: '$createdAt' }
        },
        count: { $sum: 1 }
      }
    },
    { $sort: { count: -1, '_id.year': -1, '_id.month': -1 } }
  ]).session(session)).
  then(res => assert.deepEqual(res, [
    { _id: { month: 6, year: 2018 }, count: 2 },
    { _id: { month: 6, year: 2017 }, count: 1 },
    { _id: { month: 5, year: 2017 }, count: 1 }
  ])).
  then(() => session.commitTransaction()).
  then(() => session.endSession());

Advanced Usage

Advanced users who want more fine-grained control over when they commit or abort transactions can use session.startTransaction() to start a transaction:

const Customer = db.model('Customer', new Schema({ name: String }));

let session = null;
return Customer.createCollection().
  then(() => db.startSession()).
  then(_session => {
    session = _session;
    // Start a transaction
    session.startTransaction();
    // This `create()` is part of the transaction because of the `session`
    // option.
    return Customer.create([{ name: 'Test' }], { session: session });
  }).
  // Transactions execute in isolation, so unless you pass a `session`
  // to `findOne()` you won't see the document until the transaction
  // is committed.
  then(() => Customer.findOne({ name: 'Test' })).
  then(doc => assert.ok(!doc)).
  // This `findOne()` will return the doc, because passing the `session`
  // means this `findOne()` will run as part of the transaction.
  then(() => Customer.findOne({ name: 'Test' }).session(session)).
  then(doc => assert.ok(doc)).
  // Once the transaction is committed, the write operation becomes
  // visible outside of the transaction.
  then(() => session.commitTransaction()).
  then(() => Customer.findOne({ name: 'Test' })).
  then(doc => assert.ok(doc)).
  then(() => session.endSession());

You can also use session.abortTransaction() to abort a transaction:

let session = null;
return Customer.createCollection().
  then(() => Customer.startSession()).
  then(_session => {
    session = _session;
    session.startTransaction();
    return Customer.create([{ name: 'Test' }], { session: session });
  }).
  then(() => Customer.create([{ name: 'Test2' }], { session: session })).
  then(() => session.abortTransaction()).
  then(() => Customer.countDocuments()).
  then(count => assert.strictEqual(count, 0)).
  then(() => session.endSession());