How to add columns to database results in nodejs - mysql

Question

I'm using nodejs with mysql and fs to query a database and modify the results by adding a text file to each row. I realize I need to use a promise here but I can't quite get it. I can't figure out what I'm doing wrong. Here is my code so far:
db.query(sql, params, function(err, rows) {
if(err) {
res.status(400).json({"error": err.message});
return;
}
var data = {};
var p = new Promise(function(resolve, reject) {
rows.forEach(function(row) {
var filePath = `${contentdir}/${row.strTicketNumber}-${row.strTicketRevision}.txt`;
row.fileTxt = fs.readFile(filePath, 'utf8', function(error, content) { if(error) return ""; return content; });
data[row.intSerial] = row;
});
resolve(data);
});
p.then(function() {
res.json( data );
})
});

Answer 1

You can save yourself using a Promise here if you struggle with them by using synchronous Node APIs:
db.query(sql, params, function(err, rows) {
if(err) {
res.status(400).json({"error": err.message});
return;
}
var data = {};
rows.forEach(function(row) {
var filePath = `${contentdir}/${row.strTicketNumber}-${row.strTicketRevision}.txt`;
row.fileTxt = fs.readFileSync(filePath, 'utf8');
data[row.intSerial] = row;
});
res.json( data );
});
Note readFileSync instead of readFile
Or, if you really want to use Promises, you can do it like so:
db.query(sql, params, function(err, rows) {
if(err) {
res.status(400).json({"error": err.message});
return;
}
var data = {};
var promises = []
rows.forEach(function(row) {
var filePath = `${contentdir}/${row.strTicketNumber}-${row.strTicketRevision}.txt`;
promises.push(new Promise((resolve, reject) => {
fs.readFile(filePath, 'utf8', (err, content) => {
if (err) reject(err)
row.fileTxt = content
data[row.intSerial] = row
resolve()
});
}))
});
Promise.all(promises).then(() => {
res.json( data );
})
});
The main difference between these snippets is that no. 1 reads each text file one-by-one synchronously, while no. 2 reads them "at the same time" (with a few asterisks :p)
Whether this matters to you will depend on how many files you'll be reading, as well as whether you find it significantly easier to read & maintain the first example until you are more comfortable with Promises.