NodeJS/Express/Mongoose get error Can't set headers after they are sent

Question

I'm using MEAN stack with Mongoose to query data on MongoDB. First call into /api/Validations is fine but on second time, it always gets this error:

Here my code, server.js:

var express = require('express')
    , app = express()
    , bodyParser = require('body-parser')
    , breezeRoutes = require('./breeze-routes')
    , controller = require('./controller')
    , compress = require('compression')
    , cors = require('cors')
    , errorHandler = require('./errorHandler')
    , favicon = require('static-favicon')
    , fileServer = require('serve-static')
    , http = require('http')
    , isDev = app.get('env') === 'development'
    , logger = require('morgan')
    , port = process.env["PORT"] || 4000;

app.use(favicon());
app.use(logger('dev'));
app.use(compress());
app.use(bodyParser()); // both json & urlencoded  
app.use(cors());          // enable ALL CORS requests
controller.init(app); // Configure  routes for REST API      
app.use(errorHandler);

// create server (in case interested in socket.io)
var server = http.createServer(app);

// Start listening for HTTP requests
server.listen(port); // app.listen( port ); // if we weren't using 'server'

console.log('\nListening on port ' + port);

controller.js:

(function (controller) {
    var mongooseDB = require("./mongooseDB");

    controller.init = configureRoutes;
    function configureRoutes(app) {  
        app.get('/api/Validations', getValidations);     
    }  

    function getValidations(req, res, next) {
        mongooseDB.getValidations(makeResponseHandler(res, next));
    }

    function makeResponseHandler(res, next) {
        // returns a function that handles response from a Breeze Mongo query or save
        return function (err, results) {
            if (err) {
                next(err);
            } else {
                // Prevent browser from caching results of API data requests
                //res.setHeader('Cache-Control', 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0');
                //res.setHeader("Content-Type:", "application/json");
                res.send(JSON.stringify(results));
                res.end();
            }

        }
    } 
})(module.exports)

mongooseDB.js

(function (mongooseDB) {
    var schema = require("./schema");
    var validator = require("./validator");
    var mongoUrl = 'mongodb://localhost:27017/dpt';
    var mongoose = require('mongoose');
    var Validations = mongoose.model('Validations', schema.ValidationSchema);

    mongooseDB.getValidations = function (next) {
        mongoose.connect(mongoUrl);
        mongoose.connection.on('open', function () {
            var query = Validations.find();
            query.exec(function (err, docs) {
                mongoose.disconnect();
                if (err) {
                    next(err, null);
                } else {
                    next(null, docs);
                }
            });
        });
    };
})(module.exports)

validationSchema.js

var mongoose = require('mongoose');
var Schema = mongoose.Schema;
var validationSchema = new Schema({
    ValidationID: { type: String, index: 1, required: true, unique: true },
    Integer: { type: Number, required: true },
    String: String,
    Date: Date,
    BeforeDate: Date,
    AfterDate: Date,
    Age: Number,
    CreditCard : Number,
    Email: String,
    Phone: String,
    URL: String,
    Zip: String,
    StartsWithDPT: String,
    ContainsDPT: String
}, { collection: 'Validations' });
exports.ValidationSchema = validationSchema;

I read all "can't set headers after they are sent" on stack overflow and try to apply some approach but no luck. Please help me.

UPDATE 1:

I think this is Mongoose issue because after I changed it to use MongoDB, it works fine: (function (database) { var MongoClient = require('mongodb').MongoClient;

        database.getDb = getDb;
        database.getValidations = getValidations;

        var db = null;
        // todo: get from configuration
        var mongoUrl = 'mongodb://localhost:27017/dpt';
        var mongoOptions = {
            server: { auto_reconnect: true }
        };

        function getDb(next) {
            if (db) {
                next(null, db);
            } else {
                MongoClient.connect(mongoUrl, mongoOptions, function (err, theDb) {
                    if (err) {
                        err.message = (err.message || '') + '. Is the MongoDb server running?';
                        next(err, null);
                    } else {
                        db = theDb;
                        next(null, db);
                    }
                });
            }
        }

        function getValidations(next) {
            getDb(function (err, data) {
                if (err) {
                    err.message = (err.message || '') + '. Is the MongoDb server running?';
                    next(err, null);
                } else {
                    data.collection("Validations").find().toArray(function (err1, results) {
                        next(null, results);
                    });;

                }
            });
        }    
})(module.exports)

Now I can call /api/Validations many times as I want by this issue is still there because this project requires Mongoose.

Answer 1

Instead of this:

return function (err, results) {
            if (err) {
                next(err);
            } else {
                // Prevent browser from caching results of API data requests
                //res.setHeader('Cache-Control', 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0');
                //res.setHeader("Content-Type:", "application/json");
                res.send(JSON.stringify(results));
            }
            res.end();
        }

Use this

return function (err, results) {
            if (err) {
                next(err);
            } else {
                // Prevent browser from caching results of API data requests
                //res.setHeader('Cache-Control', 'no-cache, private, no-store, must-revalidate, max-stale=0, post-check=0, pre-check=0');
                //res.setHeader("Content-Type:", "application/json");
                res.end(JSON.stringify(results));                    
            }

    }