old_contents: var MongoClient = require('mongodb').MongoClient;
  
  var url = "mongodb://localhost:27017/designiot";
  
  function MongoPersistence() {
  
  }
  
  MongoPersistence.prototype.insert = function (payload) {
      'use strict';
      MongoClient.connect(url, function (err, db) {
          var insertDocuments = function (db, callback) {
              var collection = db.collection("documents");
              collection.insert(payload, function (err, result) {
                  callback(result);
              });
          };
          insertDocuments(db, function () {
              db.close();
          });
      });
  };
  
  MongoPersistence.prototype.update = function (payload) {
      'use strict';
      MongoClient.connect(url, function (err, db) {
          var updateDocument = function (db, callback) {
              var collection = db.collection("documents");
              var topic = {user: payload.user};
              collection.update(topic, {$set: {led: payload.led}}, function (err, result) {
                  callback(result);
              });
          };
          updateDocument(db, function () {
              db.close();
          });
      });
  };
  
  MongoPersistence.prototype.query = function (queryOptions, queryCB) {
      'use strict';
      MongoClient.connect(url, function (err, db) {
          var findDocuments = function (db, query, callback) {
              var collection = db.collection("documents");
              collection.find(query, {_id: false}).toArray(function (err, docs) {
                  callback(docs);
              });
          };
  
          findDocuments(db, queryOptions, function (result) {
              db.close();
              queryCB(result);
          });
      });
  };
  
  
  module.exports = MongoPersistence;
new_contents: var MongoClient = require('mongodb').MongoClient;
  
  var url = "mongodb://localhost:27017/designiot";
  
  function MongoPersistence() {
  
  }
  
  MongoPersistence.prototype.insert = function (payload) {
      'use strict';
      MongoClient.connect(url, function (err, db) {
          var insertDocuments = function (db, callback) {
              var collection = db.collection("documents");
              collection.insert(payload, function (err, result) {
                  callback(result);
              });
          };
          insertDocuments(db, function () {
              db.close();
          });
      });
  };
  
  MongoPersistence.prototype.update = function (payload) {
      'use strict';
      MongoClient.connect(url, function (err, db) {
          var updateDocument = function (db, callback) {
              var collection = db.collection("documents");
              var topic = {user: payload.user};
              collection.update(topic, {$set: {led: payload.led}}, function (err, result) {
                  callback(result);
              });
          };
          updateDocument(db, function () {
              db.close();
          });
      });
  };
  
  MongoPersistence.prototype.query = function (queryOptions, queryCB) {
      'use strict';
      MongoClient.connect(url, function (err, db) {
          var findDocuments = function (db, query, callback) {
              var collection = db.collection("documents");
              collection.find(query).toArray(function (err, docs) {
                  callback(docs);
              });
          };
  
          findDocuments(db, queryOptions, function (result) {
              db.close();
              queryCB(result);
          });
      });
  };
  
  
  module.exports = MongoPersistence;
subject: update query method