How can I use Q promises together with MongoDB cursor.each()? - node.js

The gist of my code is as follows:
function checkWinRate(array){
var winCount = 0;
var totalCount = 0;
db.collection.find(query).each( function(foo){
if (condition){
winCount++;
}
totalCount++;
db.close();
});
return winCount/totalCount;
}
The obvious problem here is that at the return statement, both winCount and totalCount are still equal to 0. I am trying to use Q promise library to sort out a sequence of events, but I am not sure exactly how to implement it. Can anybody tell me what to do with Q?
EDIT
ID: 10T Error here. I forgot I was using the monk wrapper. For those who want to know what I did to solve this, monk has a nice way of dealing with promises in the following way:
function checkWinRate(array){
var winCount = 0;
var totalCount = 0;
var promise = db.collection.find(query).each( function(foo){
if (condition){
winCount++;
}
totalCount++;
db.close();
});
promise.success(function(){
return winCount/totalCount);
}
}

The function you are passing in each() will be executed asynchronously. You need to immediately return a promise and resolve it after all the iterations are finished:
var deferred = Q.defer();
db.collection.find(query).each(function(err, item) {
if (err) {
deferred.reject(err);
}
if (condition){
winCount++;
}
totalCount++;
if(item == null) {
// iterations are finished
deferred.resolve(winCount/totalCount);
db.close();
}
});
return deferred.promise;
To obtain value you need to call checkWinRate this way:
checkWinRate(array).then(function (rate) {});

Related

NodeJS find query inside for loop results after the loops end

var user_id = '98-XXXXXXXX'
Contact.find({user_id: user_id})
.exec(function (err, results) {
if (err) {
return next(err);
}
var finalArray = [];
for(var i = 0; i< results[0].Total; i++) {
if(results[0].Contacts[i].name != "SPAM") {
for(var j = 0; j < results[0].Contacts[i].phoneNumbers.length; j++){
var number = results[0].Contacts[i].phoneNumbers[j].number
number = number.replace(/ /g,'');
var user_id = number.substr(number.length - 10);
Login.find({user_id:user_id})
.exec(function(err,results) {
if(err) {
return next(err); }
var intCount = results.length;
if (intCount > 0)
{
console.log('called')
finalArray.push(results[0])
console.log(finalArray)
}
});
}
}
//console.log(i,results[0].Total - 1);
//if(i == results[0].Total - 1)
}
console.log('Ended Here',finalArray)
var responseTosend = {"message":finalArray,"user_id":user_id}
return res.send(responseTosend);
});
EndedHere[] this is coming up first empty, after that i got the result of login.find query which is correct. Any ideas how to get the finalArray after all the calculation.
Thanks in advance
Since the functions are returning promises within the loops, the code execution has to wait till all those promises are resolved. Consider using Promise.all or Promise.map to wait. Reference
As already mentioned, a structure like this, will not return the results, but the intermediate functions or objects before they are finished, since nodejs does not know it should await the results first.
const x = [1,2,3]
let results = []
for (let i = 0; i < x.length; i++){
results.push(someAsyncJobLikeADatabaseCall(x[i]))
}
// this will not return the results, but the intermediate async objects/functions
console.log(results)
Here is a better version using promises and the .map function. Notice, how we replaced the for loop with .map() (which you could see as a shorthand for .forEach + push() or for() + push(). Mongoose returns Promises if configured right, so you don't even have to manually define them and we can directly return them in .map.
const x = [1,2,3]
let results = []
async function getAsyncResults(array){
// map returns an array, this time, an array of promises
const promises = x.map(number => someAsyncJobLikeADatabaseCall(number))
// Promise.all resolves, if all promises in the array have been resolved
return Promise.all(promises)
}
try {
let results = await getAsyncResults(x)
// this will return the results you expect.
console.log(results)
} catch (err) {
console.log('Some error', err)
}

Return value in function from a promise block

I'm trying to write a function (using WebdriverJS lib) that iterates through a list of elements, checks the names and build an xpath locator that corresponds to that name. I simplified xpath locators here, so don't pay attention.
The issues I'm facing here are:
1) Calling this function returns undefined. As far as I understand, this is because the return statement is not in its place, but:
2) Placing it in the correct place where a synchronous code would normally work, doesn't work for async promises, hence calling this function will return the same undefined, but because the return statement fires before the "driver.findElement" statement.
How should I use the return statement here, if I want to get createdTask variable as a result of calling this function?
var findCreatedTask = function() {
var createdTask;
driver.findElements(By.xpath("//div[#id='Tasks_Tab']")).then(function(tasks) {
for (var index = 1; index <= tasks.length; index++) {
driver.findElement(By.xpath("//div[#id='Tasks_Tab'][" + index + "]//div[#class='task-title']")).getText().then(function(taskTitle) {
if (taskTitle == "testName") {
createdTask = "//div[#id='Tasks_Tab'][" + index + "]";
return createdTask;
}
});
}
});
};
You could first get all the texts with promise.map and then get the position with indexOf :
var map = webdriver.promise.map;
var findCreatedTask = function() {
var elems = driver.findElements(By.xpath("//div[#id='Tasks_Tab']//div[#class='task-title']"));
return map(elems, elem => elem.getText()).then(titles => {
var position = titles.indexOf("testName") + 1;
return "//div[#id='Tasks_Tab'][" + position + "]";
});
}
Here you go, I cleaned it up a bit. This will actually return an error if one is experienced in the nested promises:
var findCreatedTask = function() {
var Promise = require('bluebird');
var createdTask;
return driver.findElements(By.xpath("//div[#id='Tasks_Tab']"))
.then(function(tasks) {
return Promise.map(tasks, function(task){
return driver.findElement(By.xpath("//div[#id='Tasks_Tab'][" + index + "]//div[#class='task-title']")).getText()
}).then(function(taskTitles){
for (let i = 0; i < taskTitles.length; i++){
if(taskTitles[i] === 'testName'){
createdTask = "//div[#id='Tasks_Tab'][" + i + "]";
return createdTask;
}
}
});
});
};
You call it using
findCreatedTask.then(function(res){
//do your thing
}).catch(function(err){
console.error(err.stack);
});
You will not be able to return the value that you want from this function because when this function returns, the value is not defined yet.
This is not a problem that you try to return the value in the wrong place, but that you try to access it at the wrong time.
You have two options: you can either return a promise from this function, or this function can take a callback that would be called when the value is available.
Examples
This is not tested but should give you an idea on how to think about it.
Promise
Version with promise:
var findCreatedTask = function (callback) {
var createdTask;
return new Promise(function (resolve, reject) {
driver.findElements(By.xpath("//div[#id='Tasks_Tab']")).then(function(tasks) {
for (let index = 1; index <= tasks.length && !createdTask; index++) {
driver.findElement(By.xpath("//div[#id='Tasks_Tab'][" + index + "]//div[#class='task-title']")).getText().then(function(taskTitle) {
if (taskTitle == "testName") {
createdTask = "//div[#id='Tasks_Tab'][" + index + "]";
resolve(createdTask);
}
});
}
});
});
};
and then you call it with:
findCreatedTask().then(function (createdTask) {
// you have your createdTask here
});
Callback
Version with callback:
var findCreatedTask = function (callback) {
var createdTask;
driver.findElements(By.xpath("//div[#id='Tasks_Tab']")).then(function(tasks) {
for (let index = 1; index <= tasks.length && !createdTask; index++) {
driver.findElement(By.xpath("//div[#id='Tasks_Tab'][" + index + "]//div[#class='task-title']")).getText().then(function(taskTitle) {
if (taskTitle == "testName") {
createdTask = "//div[#id='Tasks_Tab'][" + index + "]";
callback(null, createdTask);
}
});
}
});
};
and then you call it with:
findCreatedTask(function (err, createdTask) {
// you have your createdTask here
});
More info
You can read some other answers that explain how promises and callbacks work if you're interested to know ore about it:
A detailed explanation on how to use callbacks and promises
Explanation on how to use promises in complex request handlers
An explanation of what a promise really is, on the example of AJAX requests
An explanation of callbacks, promises and how to access data returned asynchronously

callback is not a function node js

I am new to javascript and i am having trouble solving this error. I get the message: "callback is not a function" at:"return callback(rolesArray)".
Rol.getAllRoles = function(callback){
sql = "select role from Role;";
var rolesArray = [];
var role;
mysql.connection(function(err,conn){
if (err){
return callback(err);
}
conn.query(sql,function(err,rows){
if (err){
return callback(err);
}
for(var i=0; i < rows.length; i++){
role = rows[i].role;
rolesArray.push(rol);
}
console.log("roles: " + rolesArray);
return callback(rolesArray);
});
});
}
The console.log outputs:"roles: admin,customer" so the connection with the database works.
That error means that you are not passing a function to Rol.getAllRoles(fn) when you call it.
In addition, so that you can have proper error handling in your callback and so you can more easily distinguish between an error and the actual data, you should always pass a first argument to the callback that indicates whether there was an error or not and then the second argument (if not an error) can be your results array like this:
Rol.getAllRoles = function(callback){
sql = "select role from Role;";
var rolesArray = [];
var role;
mysql.connection(function(err,conn){
if (err){
return callback(err);
}
conn.query(sql,function(err,rows){
if (err){
return callback(err);
}
for(var i=0; i < rows.length; i++){
role = rows[i].role;
rolesArray.push(rol);
}
console.log("roles: " + rolesArray);
// make sure the first argument to the callback
// is an error value, null if no error
return callback(null, rolesArray);
});
});
}
And, then you should be calling it like this:
Rol.getAllRoles(function(err, rolesArray) {
if (err) {
// handle error here
} else {
// process rolesArray here
}
});
This style of calling an async callback as in callback(err, data) is a very common async callback design pattern. It allows all callers to see if there was an error or not and if there was no error to get access to the final result.
I'd suggest the following:
Rol.getAllRoles = function(callback){
var sql = "select role from Role;";
var rolesArray = [];
var role;
callback = callback || function(){};
mysql.connection(function(err,conn){
if (err){
return callback(err);
}
conn.query(sql,function(err,rows){
if (err){
return callback(err);
}
for(var i=0; i < rows.length; i++){
role = rows[i].role;
rolesArray.push(rol);
}
console.log("roles: " + rolesArray);
return callback(rolesArray);
});
});
}
This way you enforce that callback is always a function. If you run it like Rol.getAllRoles() then you would get an error previously. Now you wont. You wont get any data back though.
Make sure you are calling Rol.getAllRoles with the proper parameter (ie: a function).

Synchronous for loop in node js

So let's say I have the following for loop
for(var i = 0; i < array.length; i++){
Model.findOne({ _id = array[i].id}, function(err, found){
//Some stuff
});
}
How do I make this code work? Every time I run it I get array[i] = undefinedbecause the mongo-db query is asynchronous and the loop has already iterated 5 times by the time the first query is even completed. How do I go about tackling this issue and waiting for the query to complete before going on to the next iteration?
This doesn't specifically answer your question, but addresses your problem.
I'd use an $in query and do the filtering all at once. 20 calls to the db is pretty slow compared to 1:
// grab your ids
var arrayIds = myArray.map(function(item) {
return item._id;
});
// find all of them
Model.find({_id: {$in: arrayIds}}, function(error, foundItems) {
if (error) {
// error handle
}
// set up a map of the found ids
var foundItemsMap = {};
foundItems.forEach(function(item) {
foundItemsMap[item._id] = true;
});
// pull out your items that haven't been created yet
var newItems = [];
for (var i = 0; i < myArray.length; i++) {
var arrayItem = myArray[i];
if ( foundItemsMap[arrayItem._id] ) {
// this array item exists in the map of foundIds
// so the item already exists in the database
}
else {
// it doesn't exist, push it into the new array
newItems.push(arrayItem);
}
}
// now you have `newItems`, an array of objects that aren't in the database
});
One of the easiest ways to accomplish something like you want is using promises. You could use the library q to do this:
var Q = require('q');
function fetchOne(id) {
var deferred = Q.defer();
Model.findOne({ _id = id}, function(err, found){
if(err) deferred.reject(err);
else deferred.resolve(found);
});
return deferred.promise;
}
function fetch(ids, action) {
if(ids.length === 0) return;
var id = ids.pop();
fetchOne(id).then(function(model) {
action(model);
fetch(ids, action);
});
}
fetch([1,2,3,4,5], function(model) { /* do something */ });
It is not the most beautiful implementation, but I'm sure you get the picture :)
Not sure if this is the right way, it could be a bit expensive but this how i did it.
I think the trick is to pull all your data and then looking for an id match.
Model.find(function(err, data) {
if (err) //handle it
for (var i=0; i<array.length; i++) {
for (var j=0; ij<data.length; j++) {
if(data[j].id == array[i].id) {
// do something
}
}
}
}

NodeJS + redis gives weird results

Maybe the results ain't weird, but I started using Node 1-2 months ago so for me they are...
I have a loop which sorts out every other value of the array returned by hgetall (Redis command) and in that loop I call a function to get all values from another table with keys stored in the sorted array. This was more difficult to explain than I thought. Here's my code:
Pastebin: http://pastebin.com/tAVhSUV1 (or see below)
function getInfo (cn, callback) {
var anArray = [];
redis_client.hgetall('chat_info:' + cn, function (err, vals) {
if(err) { throw err; }
for(i in vals) {
anArray.push(vals[i]);
}
return callback(anArray);
});
}
redis_client.hgetall('chat_rooms:' + POST.chat_name, function (err, val) {
if(err) { throw err; }
var vars = [],
rArr = [];
for (i in val) {
vars.push(i);
}
for(var i = 0; i < vars.length; i += 1) {
if(i%2 === 0) {
getInfo(vars[i], function (hej) {
rArr.push(hej);
});
}
}
});
The callback from the call to getInfo() is executed after the entire loop. Am I missing out on something here? Because it can't do that, right? (when I use rArr (right after the loop) it's empty, nbBut if I log it in the callback it gets logged after everything else written after the loop)
Yes, that's probably normal.
Understand that callbacks are executed after the hgetall call. Which mean that when the redis functions receive somehting it will call the callbacks. In other words, all the callbacks can be executed later.
As javascript only works in one thread, the calls to hgetall should be blocking to be executed as they come in the for loop. But as you're more certainly using async IO. The for loop ends and then it will start calling each callbacks that were queued inside the javascript event loop.
Edit
Unfortunately, to achieve what you're trying to do, you should wrap your code inside many other callbacks. You can use this project to make it easier: https://github.com/caolan/async
You should be able to install it using npm install async.
You'd have to do something like that:
function getInfo (cn) {
return function(callback) {
var anArray = [];
redis_client.hgetall('chat_info:' + cn, function (err, vals) {
if(err) { throw err; }
for(i in vals) {
anArray.push(vals[i]);
}
return callback(anArray);
});
};
}
redis_client.hgetall('chat_rooms:' + POST.chat_name, function (err, val) {
if(err) { throw err; }
var vars = [],
rArr = [],
callbacks = [];
for (i in val) {
vars.push(i);
}
for(var i = 0; i < vars.length; i += 1) {
if(i%2 === 0) {
callbacks.push(getInfo(vars[i]));
}
}
async.series(callbacks, function (err, results) {
// Final code here
});
});

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