Querying multiple MongoDB collections in Node.js asynchronously - node.js

Question

I need to fetch two different MongoDB collections (db.stats and db.tables ) for the same request req.
Now, in the code below, I am nesting the queries within the callback function.
router.post('/', (req, res) => {
let season = String(req.body.year);
let resultData, resultTable;
db.stats.findOne({Year: season}, function (err, data) {
if (data) {
resultData = getResult(data);
db.tables.findOne({Year: season}, function (err, data) {
if (data) {
resultTable = getTable(data);
res.render('index.html', {
data:{
result : resultData,
message: "Working"}
});
} else {
console.log("Error in Tables");
}
});
} else {
console.log("Error in Stats");
}
});
});
This code works, but there a few things that don't seem right. So my question is:
How do I avoid this nested structure? Because it not only looks ugly but also, while I am processing these requests the client side is unresponsive and that is bad.

Answer 1

What you have right now is known as the callback hell in JavaScript. This is where Promises comes in handy.
Here's what you can do:
router.post('/', (req, res) => {
let season = String(req.body.year);
var queries = [
db.stats.findOne({ Year: season }),
db.tables.findOne({ Year: season })
];
Promise.all(queries)
.then(results => {
if (!results[0]) {
console.log("Error in Stats");
return; // bad response. a better way is to return status 500 here
} else if (!results[1]) {
console.log("Error in Tables");
return; // bad response. a better way is to return status 500 here
}
let resultData = getResult(results[0]);
let resultTable = getTable(results[1]);
res.render('index.html', { data: {
result : resultData,
message: "Working"
} });
})
.catch(err => {
console.log("Error in getting queries", err);
// bad response. a better way is to return status 500 here
});
});
It looks like you are using Mongoose as your ODM to access your mongo database. When you don't pass in a function as the second parameter, the value returned by the function call (e.g. db.stats.findOne({ Year: season })) will be a Promise. We will put all of these unresolved Promises in an array and call Promise.all to resolve them. By using Promise.all, you are waiting until all of your database queries get executed before moving on to render your index.html view. In this case, the results of your database function calls will be stored in the results array in the order of your queries array.
Also, I would recommend doing something like res.status(500).send("A descriptive error message here") whenever there is an error on the server side in addition to the console.log calls.
The above will solve your nested structure problem, but latter problem will still be there (i.e. client side is unresponsive when processing these requests). In order to solve this, you need to first identify your bottleneck. What function calls are taking up most of the time? Since you are using findOne, I do not think that will be the bottleneck unless the connection between your server and the database has latency issues.
I am going to assume that the POST request is not done through AJAX since you have res.render in it, so this problem shouldn't be caused by any client-sided code. I suspect that either one of getResult or getTable (or both) is taking up quite a significant amount of time, considering the fact that it causes the client side to be unresponsive. What's the size of the data when you query your database? If the size of it is so huge that it takes a significant amount of time to process, I would recommend changing the way how the request is made. You can use AJAX on the front-end to make a POST request to the back-end, which will then return the response as a JSON object. That way, the page on the browser would not need to reload, and you'll get a better user experience.

Answer 2

mongodb driver return a promise if you dont send a callback so you can use async await
router.post('/', async(req, res) => {
let season = String(req.body.year);
let resultData, resultTable;
try {
const [data1,data2] = await Promise.all([
db.stats.findOne({Year: season}),
db.tables.findOne({Year: season})
]);
if (data1 && data2) {
resultData = getResult(data1);
resultTable = getTable(data2);
return res.render('index.html', {
data: {
result: resultData,
message: "Working"
}
});
}
res.send('error');
console.log("Error");
} catch (err) {
res.send('error');
console.log("Error");
}
});