Notes on Hooks, Middleware in Mongoose

[natac13]

Pre Hooks

• can have serial or parallel hooks

// user is an instance of Model, which are mongoose documents that have a one-to-one mapping to a record in the database
const user = new SchemaModel( ... );

// serial hooks
user.pre('save', function(next) {
  // do stuff
  next();
});

// parallel hooks
user.pre('save', true, function(next, done) {
  // do synchronous stuff
  // then pass control to next middleware/DB
  next();

  // do **a**synchronous stuff
 setTimeout(done, 100000);
});

Use Case

• removing a user from the database will remove all his post.
• deleting contact could block them as well...
  If there is an error that I want to propagate then pass in a new Error() to the next() function.
  Then when the user doc calls save() then the error will show up in the callback/promise

user.save()
  .then(function fulfilled() {... })
  .catch(function handleError(error) { 
  // this is where Ill find the error passed to next()
  console.log(error); 
} 

Post Hooks

• no flow control (done())
• can still happen in a serial fashion, by giving next() as second parameter
• a save() call triggers the 'validate' hook. Note this cause all pre & post 'validate' hooks to happen before any saving to the database occurs.

user.post('save', function(doc) {
  // Do stuff after save
}

// can chain them 
user.post('save', function(doc, next) {
  // do stuff then pass to next post hook
  next();
}

Important

• hooks and not triggered with update() and findOneAndUpdate()
• Mongoose 4.0 has specific hooks for these

user.pre('update', function() {...});
user.pre('find', function() {...});