How to properly throw and handle errors in promises in Sails.js? - promise

Question

I'm starting to convert my callback code to promises in Sails.js, but I don't understand how I can raise custom errors and handle them in the promise chain. Sails.js uses Q as its promise library.
User.findOne({email: req.param('professorEmail'), role: 'professor'})
.then(function (user) {
if (user) {
return Course.create({
user_id: user.id,
section: req.param('section'),
session: req.param('session'),
course_code: req.param('course_code')
});
} else {
// At this point `user` is undefined which means that no professor was found so I want to throw an error.
// Right now the following statement does throw the error, but it crashes the server.
throw new Error('That professor does not exist.');
// I want to be able to handle the error in the .fail() or something similar in the promise chain.
}
}).then(function (createSuccess) {
console.log(createSuccess);
}).fail(function (err) {
console.log(err);
});
Right now the .fail() is never called because the thrown error crashes the server.

Answer 1

Use .catch() instead of .fail().

Answer 2

Waterline's claim complete Q promise object after the first then seems untrue by your test. I've verified it myself as well and found a workaround.
You can do this :
var Q = require('q');
[...]
Q(User.findOne({email: req.param('professorEmail'), role: 'professor'}))
.then(function (user) {
if (user) {
return Course.create({
user_id: user.id,
section: req.param('section'),
session: req.param('session'),
course_code: req.param('course_code')
});
} else {
// At this point `user` is undefined which means that no professor was found so I want to throw an error.
// Right now the following statement does throw the error, but it crashes the server.
throw new Error('That professor does not exist.');
// I want to be able to handle the error in the .fail() or something similar in the promise chain.
}
}).then(function (createSuccess) {
console.log(createSuccess);
}).fail(function (err) {
console.log(err);
});
This will return a true Q promise.