Sending express response with data from 2 different mongoose queries - node.js

Question

I need to query 2 different collections and send it in the express response. I have a very vague idea of what is needed to do so. I tried to contact the query documents to an empty array and send that new array as the response. But I receive an empty array as a response.
This is my route.
site.route('/campus/development')
.get(function(req, res) {
var devPosts = [];
development.find().exec(function(err, docs) {
if (err) {
console.log('Error : ' + err);
} else {
if (docs != null) {
devPosts = devPosts.concat(docs);
console.log(docs);
} else {
console.log('No posts found');
}
}
});
jobs.find().exec(function(err, jobs) {
if (err) {
console.log('Error : ' + err);
} else {
if (jobs != null) {
devPosts = devPosts.concat(jobs);
console.log(jobs);
} else {
console.log('No jobs');
}
}
});
res.send(devPosts);
});

Answer 1

This is due to the async operation of the requests to the database. There are a variety of solutions to this but basically distill down to two types: callbacks or promises.
A callback solution might look like:
site.route('/campus/development')
.get(function(req, res) {
development.find().exec(function(err, devDocs) {
if (err) {
console.log('Error : ' + err);
} else {
if (devDocs != null) {
console.log(devDocs);
jobs.find().exec(function(err, jobs) {
if (err) {
console.log('Error : ' + err);
} else {
if (jobs != null) {
console.log(jobs);
res.send([devDocs, jobs]);
} else {
console.log('No jobs');
}
}
});
} else {
console.log('No posts found');
}
}
});
});
But this introduces a couple of interesting issues: one is the phenomenon known as callback hell and the other is that you should be responding with the errors which means you would need to have a response call for each error (albeit this is a very simplistic approach to it).
As mentioned earlier there is another type of solution which involves using promises. There are a bunch of libraries that you can use and actually Mongoose returns a promise from the exec method. However if you are on Node 0.12.x you can also use the native Promise (it was introduced in 0.11 but you should be using 0.12.x over 0.11.x). A benefit to using the native promise over the one returned from Mongoose is that you can execute these requests in parallel since they don't depend on each other.
site.route('/campus/development')
.get(function(req, res) {
Promise.all([
development.find().exec(), // returns a promise
jobs.find().exec() // returns a promise
]).then(function(results) {
// results is [devDocs, jobs]
console.log(results);
res.send(results);
}).catch(function(err) {
res.send(err);
});
});