Fork me on GitHub

Middleware

Middleware (also called pre and post hooks) are functions which are passed control during execution of asynchronous functions. Middleware is specified on the schema level and is useful for writing plugins. Mongoose 4.0 has 2 types of middleware: document middleware and query middleware. Document middleware is supported for the following document functions.

Query middleware is supported for the following Model and Query functions.

Both document middleware and query middleware support pre and post hooks. How pre and post hooks work is described in more detail below.

Pre

There are two types of pre hooks, serial and parallel.

Serial

Serial middleware are executed one after another, when each middleware calls next.

var schema = new Schema(..);
schema.pre('save', function (next) {
  // do stuff
  next();
});

Parallel

Parallel middleware offer more fine-grained flow control.

var schema = new Schema(..);
schema.pre('save', true, function (next, done) {
  // calling next kicks off the next middleware in parallel
  next();
  doAsync(done);
});

The hooked method, in this case save, will not be executed until done is called by each middleware.

Use Cases

Middleware are useful for atomizing model logic and avoiding nested blocks of async code. Here are some other ideas:

Error handling

If any middleware calls next or done with an error, the flow is interrupted, and the error is passed to the callback.

schema.pre('save', function (next) {
  var err = new Error('something went wrong');
  next(err);
});

// later...

myDoc.save(function (err) {
  console.log(err.message) // something went wrong
});

Post middleware

post middleware are executed after the hooked method and all of its pre middleware have completed. post middleware do not directly receive flow control, e.g. no next or done callbacks are passed to it. post hooks are a way to register traditional event listeners for these methods.

schema.post('init', function (doc) {
  console.log('%s has been initialized from the db', doc._id);
})
schema.post('validate', function (doc) {
  console.log('%s has been validated (but not saved yet)', doc._id);
})
schema.post('save', function (doc) {
  console.log('%s has been saved', doc._id);
})
schema.post('remove', function (doc) {
  console.log('%s has been removed', doc._id);
})

Notes on findAndUpdate() and Query Middleware

Pre and post save() hooks are not executed on update(), findOneAndUpdate(), etc. You can see a more detailed discussion why in this GitHub issue. Mongoose 4.0 has distinct hooks for these functions.

schema.pre('update', function() {
  this.start = Date.now();
});

schema.post('update', function() {
  console.log('update() took ' + (Date.now() - this.start) + ' millis');
});

Next Up

Now that we've covered middleware, let's take a look at Mongoose's approach to faking JOINs with its query population helper.