Loop through MySQL rows and store results in array - mysql

Question

I am trying to store details of affectedRows from a MySQL INSERT query using NodeJS. My mind is melting trying to comprehend callbacks and Promises. Being a single-man dev team I wanted to reach out and ask for the clearest explanation as to how a callback can be applied here in a foreach loop.
The goal should be clear from these few lines of code; store data in the affected_rows[] array.
var affected_rows = [];
asset_array.forEach(function(asset) { // Populate the asset table
var query_string = "INSERT IGNORE INTO " + asset_table + " SET symbol = '" + asset[0] + "', name = '" + asset[1] + "'";
connection.query(query_string, function(err, rows, fields) {
if (err) throw err;
if ( rows.affectedRows > 0 ) {
data_to_push = [asset_table, asset[0], asset[1]];
affected_rows.push(data_to_push);
}
});
});
console.log(affected_rows); // [] for obvious async reasons

Answer 1

One option would be to process the asset_array inside a function and pass a callback into it and when loops through asset_array check if the current index matches the asset_array length (-1). If so call the callback.
var affected_rows = [];
function processAssets(cb) {
var array_len = asset_array_len.length
asset_array.forEach(function(asset, index) {
var query_string = 'INSERT IGNORE INTO ' + asset_table + ' SET symbol = \'' + asset[0] + '\', name = \'' + asset[1] + '\'';
connection.query(query_string, function(err, rows, fields) {
if (err) throw err
if (rows.affectedRows > 0) {
data_to_push = [asset_table, asset[0], asset[1]];
affected_rows.push(data_to_push);
}
if (index === (array_len - 1)) cb()
});
});
}
processAssets(function() {
console.log(affected_rows)
})

Answer 2

Will suggest you to have a look at async Queue.
You can change your code like this to use it.
//2nd Step - Perform each task and then call callback() to move to next task
var q = async.queue(function(query_string, callback) {
connection.query(query_string, function(err, rows, fields) {
if (err) throw err;
if ( rows.affectedRows > 0 ) {
data_to_push = [asset_table, asset[0], asset[1]];
affected_rows.push(data_to_push);
}
callback(); //call next task
});
}, 2); //here 2 means concurrency ie 2 tasks will run in parallel
//Final Step - Drain gives you end of queue which means all tasks have finished processing
q.drain = function() {
//Do whatever you want after all tasks are finished
};
//1st Step - create a queue of all tasks that you need to perform
for (var i = 0; i < asset_array.length ; i++) {
var query_string = "INSERT IGNORE INTO " + asset_table + " SET symbol = '" + asset[0] + "', name = '" + asset[1] + "'";
q.push(query_string);
}