How to make multiple mysql queries in Node with promises - mysql

Question

G'day all,
I'm trying to convert some old php code over to Node, and part of the journey has been trying to figure out the best way to perform sql queries against my database (I'm using SQL so I can port the existing database over).
I've got them working, but have encountered the "Pyramid of Doom" problem, and it's subsequent scope issues (i.e. the returned values not baing available to subsequent "then"s).
An example of the sort of code I have here is: (dbPool.queryOPromise returns a query wrapped in a promise)
dbPool.queryOPromise(query)
.then(function(result){
console.log(result);
var query = {
sql:"INSERT INTO newusers (newuserid, ipaddress, email) VALUES (?,?,?)",
values: [newuserid, ipAddress, email]
};
dbPool.queryOPromise(query)
.then(function(value){
console.log(value);
if(value.code==200) {
res.status(200).json({code:200, status:"New User Created"});
} else {
res.status(400).json({code:value.code, status:"Error creating new user: ".value.status});
}
})
})
Does anyone have a view on the best way to attack this situation?
Thanks!

Answer 1

You're supposed to return the subsequent promises instead of calling .then on them
dbPool.queryOPromise(query)
.then(function(result) {
console.log(result);
var query = {
sql: "INSERT INTO newusers (newuserid, ipaddress, email) VALUES (?,?,?)",
values: [newuserid, ipAddress, email]
};
// RETURN the second promise,
return dbPool.queryOPromise(query);
})
.then(function(value) {
console.log(value);
if (value.code == 200) {
res.status(200).json({code: 200, status: "New User Created"});
} else {
res.status(400).json({code: value.code, status: "Error creating new user: ".value.status });
}
})
.catch(console.error); // and always catch the errors at the end.
It's a #1 rookie mistake in using promises. Checkout this wonderfully written article addressing issues exactly like this

Answer 2

For node -v > 8.x only,
Share my working example:
I use this Promisified MySQL middleware for Node.js
read this article Create a MySQL Database Middleware with Node.js 8 and Async/Await
here is my database.js
var mysql = require('mysql');
// node -v must > 8.x
var util = require('util');
// !!!!! for node version < 8.x only !!!!!
// npm install util.promisify
//require('util.promisify').shim();
// -v < 8.x has problem with async await so upgrade -v to v9.6.1 for this to work.
// connection pool https://github.com/mysqljs/mysql [1]
var pool = mysql.createPool({
connectionLimit : process.env.mysql_connection_pool_Limit, // default:10
host : process.env.mysql_host,
user : process.env.mysql_user,
password : process.env.mysql_password,
database : process.env.mysql_database
})
// Ping database to check for common exception errors.
pool.getConnection((err, connection) => {
if (err) {
if (err.code === 'PROTOCOL_CONNECTION_LOST') {
console.error('Database connection was closed.')
}
if (err.code === 'ER_CON_COUNT_ERROR') {
console.error('Database has too many connections.')
}
if (err.code === 'ECONNREFUSED') {
console.error('Database connection was refused.')
}
}
if (connection) connection.release()
return
})
// Promisify for Node.js async/await.
pool.query = util.promisify(pool.query)
module.exports = pool
You must upgrade node -v > 8.x
you must use async function to be able to use await.
example:
var pool = require('./database')
// node -v must > 8.x, --> async / await
router.get('/:template', async function(req, res, next)
{
...
try {
var _sql_rest_url = 'SELECT * FROM arcgis_viewer.rest_url WHERE id='+ _url_id;
var rows = await pool.query(_sql_rest_url)
_url = rows[0].rest_url // first record, property name is 'rest_url'
if (_center_lat == null) {_center_lat = rows[0].center_lat }
if (_center_long == null) {_center_long= rows[0].center_long }
if (_center_zoom == null) {_center_zoom= rows[0].center_zoom }
_place = rows[0].place
} catch(err) {
throw new Error(err)
}