Error handling with Mongoose

Question

I am an absolute NodeJS beginner and want to create a simple REST-Webservice with Express and Mongoose.

Whats the best practice to handle errors of Mongoose in one central place?

When anywhere an database error occurs I want to return a Http-500-Error-Page with an error message:

if(error) {
  res.writeHead(500, {'Content-Type': 'application/json'});
  res.write('{error: "' + error + '"}');
  res.end();
}

In the old tutorial http://blog-next-stage.learnboost.com/mongoose/ I read about an global error listener:

Mongoose.addListener('error',function(errObj,scope_of_error));

But this doesn't seem to work and I cannot find something in the official Mongoose documentation about this listener. Have I check for errors after every Mongo request?

Answer 1

If you're using Express, errors are typically handled either directly in your route or within an api built on top of mongoose, forwarding the error along to next.

app.get('/tickets', function (req, res, next) {
  PlaneTickets.find({}, function (err, tickets) {
    if (err) return next(err);
    // or if no tickets are found maybe
    if (0 === tickets.length) return next(new NotFoundError));
    ...
  })
})

The NotFoundError could be sniffed in your error handler middleware to provide customized messaging.

Some abstraction is possible but you'll still require access to the next method in order to pass the error down the route chain.

PlaneTickets.search(term, next, function (tickets) {
  // i don't like this b/c it hides whats going on and changes the (err, result) callback convention of node
})

As for centrally handling mongoose errors, theres not really one place to handle em all. Errors can be handled at several different levels:

connection errors are emitted on the connection your models are using, so

mongoose.connect(..);
mongoose.connection.on('error', handler);

// or if using separate connections
var conn = mongoose.createConnection(..);
conn.on('error', handler);

For typical queries/updates/removes the error is passed to your callback.

PlaneTickets.find({..}, function (err, tickets) {
  if (err) ...

If you don't pass a callback the error is emitted on the Model if you are listening for it:

PlaneTickets.on('error', handler); // note the loss of access to the `next` method from the request!
ticket.save(); // no callback passed

If you do not pass a callback and are not listening to errors at the model level they will be emitted on the models connection.

The key take-away here is that you want access to next somehow to pass the error along.