Nodejs multiple sql query loop - mysql

Question

I am pretty new to nodejs and async worlds.
The case is, I have an array like var ids = [1, 2, 3, 4];
I need to update mytable according to sequence of the array element. So I do something like:
sort: function(ids, callback) {
// dont worry about this
this.create_connection();
this.connection.connect();
for(var i=0; i<ids.length;i++) {
var q = "UPDATE mytable SET sequence="+i+" where id="+ids[i]+"; ";
this.connection.query(q, function(err, result) {
// I am not sure about this
// callback(err);
});
}
// I need to return callback at the end
// return callback();
this.connection.end();
}
But yes.. it does not work because I have to return callback.. I think I need to do the query syncronously.. I am not sure. Please help thanks.

Answer 1

If you are new to async worlds, you should take a look at module 'async'.
You can then do something like this :
async.forEachOfSeries(ids, function(id,index,callback){
var q = "UPDATE mytable SET sequence="+index+" where id="+id+"; ";
this.connection.query(q, function(err, result) {
callback();
});
},function done(){
// whatever you want to do onces all the individual updates have been executed.
})

Answer 2

See my inline comments:
sort: function(ids, callback) {
this.create_connection();
this.connection.connect();
var q = "UPDATE mytable SET sequence CASE id ";
// Don't execute one query per index in ids - that's inefficient
// Instead, pack up all the queries and execute them at once
for(var i=0; i<ids.length;i++) {
q += "WHEN " + ids[i] + " THEN " + i + " ";
}
q += "ELSE sequence END;";
// The sort method will return the result of connection.query
return this.connection.query(q, function(err, result) {
// End the connection
this.connection.end();
if(err) {
// Handle any error here
return callback(err);
}
// Otherwise, process, then return the result
return callback(err, result);
});
}
And here's something slightly more elegant:
sort: function(ids, callback) {
this.create_connection();
this.connection.connect();
// Don't execute one query per index in ids - that's inefficient
// Instead, pack up all the queries and execute them at once
var q = ids.reduce(function(pv, cv, ci){
return pv + " WHEN " + cv + " THEN " + ci + " ";
}, "UPDATE mytable SET sequence CASE id ") + " ELSE sequence END;";
// The sort method will return the result of connection.query
return this.connection.query(q, function(err, result) {
// End the connection
this.connection.end();
if(err) {
// Handle any error here
return callback(err);
}
// Otherwise, process, then return the result
return callback(err, result);
});
}
And you can replace the .reduce in the previous example with the following, if you want to use ES6 arrow functions:
var q = ids.reduce((pv, cv, ci) => pv + " WHEN " + cv + " THEN " + ci + " ",
"UPDATE mytable SET sequence CASE id ") + " ELSE sequence END;";