Getting Error with express : Can't set headers after they are sent

Question

I am creating express app and in the router I am going to fetch data from the mongodb. here is my code

router.get('/', function(req, res, next) {
    MongoClient.connect(url, function(err, db) {
    db.collection('school').find({}).toArray(function(err, doc) {
          assert.equal(null, err);
          assert.ok(doc != null);
          res.render('index', { title: 'iGyan.org', schools: doc});
        });

    db.collection('students').find({}).toArray(function(err, doc) {
          assert.equal(null, err);
          assert.ok(doc != null);
          res.render('index', { title: 'iGyan.org',students: doc});
        });
    db.close();
});
});

When I run the code and opens a url in browser, it gives me error on the console stating

Error: Can't set headers after they are sent.

I have seen almost all suggested question asked on stack overflow, but cant get help out of it.

I know the error is because I am rendering the res two time in the function but do not know how to overcome the situation.

Answer 1

Since the db calls are async, you need to make sure they're both done before rendering the response. Based on that answer , you could use the async lib to accomplish this.

You could render the response only once like this:

router.get('/', function(req, res, next) {

    MongoClient.connect(url, function(err, db) {
        var data = {};
        var tasks = [
            // Load users
            function(callback) {
                db.collection('school').find({}).toArray(function(err, doc) {
                    if (err) return callback(err);
                    data.schools = doc;
                    callback();
                });
            },
            // Load colors
            function(callback) {
                db.collection('students').find({}).toArray(function(err, doc) {
                    if (err) return callback(err);
                    data.students: doc;
                    callback();
                });
            }
        ];

        //This function gets called after the two tasks have called their "task callbacks"
        async.parallel(tasks, function(err) {
            //If an error occurred, let express handle it by calling the `next` function
            if (err) return next(err);
            db.close();
            res.render('index', {
                title: 'iGyan.org',
                students: data.students,
                schools: data.schools
            });
        });
    });

});

Source: Fetch from multiple, separate, collections with Express and MongoDB