NODEJS node-mysql cannot pass query results in nested queries - mysql

Question

I am using node, angular and mysql, the node routes would return a json that would be processed by angular, the json is returned by first querying the mysql DB using the node-mysql module,
In the below code I am unable to set the value of CreatedID, but the value gets logged properly in terminal. I was facing the same issue in the 1st query but then sorted it in the below code, now unable to access the nested query results.
var mysql = require('node-mysql/node_modules/mysql');
var connection = mysql.createConnection({
host : 'localhost',
user : "root",
password: "",
database:'designtaskmanager'
});
connection.connect();
var allDbCalls = function() {
var sendData = {};
var rowData = {};
var temp={};
var _this = this;
this.sendTask = function(callback) {
module.exports.taskData = rowData;
callback['success']();
};
this.getTask = function(callback) {
var strQuery = "select * from task";
connection.query( strQuery, function(err, rows){
if(err)
{
callback['failure']();
throw err;
}
else
{
//rowData = rows;
var tasks=[];
for (var i in rows)
{
var Title = rows[i].task_title;
var TaskDescription=rows[i].task_description;
var TaskCategory=rows[i].task_category;
var TaskID=rows[i].task_id;
var TaskStatus=rows[i].task_status;
var TaskStatusMessage
var CreatedBy;
var TaskCreationDate=rows[i].task_creation_date;
var _MS_PER_DAY = 1000 * 60 * 60 * 24;
var currentdate = new Date();
var ddd=dateDiffInDays(TaskCreationDate,currentdate);
function dateDiffInDays(a, b) {
// Discard the time and time-zone information.
var utc1 = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
var utc2 = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate());
return Math.floor((utc2 - utc1) / _MS_PER_DAY);
}
if(TaskStatus==0)
{
TaskStatus="label-info";
TaskStatusMessage="Ongoing since";
}
else if(TaskStatus==1)
{
TaskStatus="label-default";
TaskStatusMessage="Paused since"
}
else if(TaskStatus==2)
{
TaskStatus="label-success";
TaskStatusMessage="Completed in"
}
//USER DETAILS QUERY
var crid=rows[i].task_created_by;
var creatorQuery = "select user_email from users where user_id like ?";
connection.query( creatorQuery,[crid], function(err, createdbyrows){
if(err)
{
callback['failure']();
throw err;
}
else
{
for(var j=0; j< createdbyrows.length;j++)
{
CreatedBy=createdbyrows[0].user_email;
console.log(j);
}
console.log(CreatedBy);
}
});
var taskItem={"TaskID":TaskID,"TaskTitle":Title,"TaskDescription":TaskDescription,"TaskCategory":TaskCategory,"CreatedBy":CreatedBy,"TaskStatus":TaskStatus,"TaskStatusMessage":TaskStatusMessage,"DifferenceInDays":ddd};
tasks.push(taskItem);
}
rowData=tasks;
_this.sendTask(callback);
}
});
}
}
module.exports = function () {
var instance = new allDbCalls();
return instance;
};

Answer 1

The reason that you're seeing it on the console but not in the callback is due to a misunderstanding of asynchronous programming. When you:
for(var i in rows) {}
You are actually queuing up all of those queries at the same time, then, immediately after you try to set rowData to an empty array:
rowData=tasks; // remember, none of the queries have finished yet
_this.sendTask(callback);
So you pretty much call your callback when tasks is still an empty array. Remember, you can't call your final callback until ALL of your nested queries have finished!
To accomplish this, you may want to look at the async library: https://github.com/caolan/async#eachSeries
This will help you accomplish what you really want.
var async = require("async");
async.eachSeries(rows, function(row, cb) {
// Do each query here
// then call cb() when done, which tells the async library
// to "go to the next item in the array"
}, function(err) {
// This will get called when all of the single queries are finished
// Check err, then call your callback
_this.sendTask(callback);
});