Synchronous mysql in Node.js - mysql

Question

I know that node.js is event driven and I should do this async but i can't find in my mind a way to do it.
so, i have this
var querystr = "SELECT * FROM groups";
var result = "";
dbClient.query(querystr, function (err, res, fields) {
if (err) {
console.log(err);
return;
}
for(var i in res) {
result = result + "#" +res[i].name + " =";
for (var j in res[i].members.split(",")) {
var memberquery;
if (j.substr(0,1) == "#") {
memberquery = "SELECT name FROM groups WHERE id = "+j.substr(1, j.length-1);
} else {
memberquery = "SELECT username FROM users WHERE id= "+j;
}
dbClient.query(memberquery, function(err, memres, fields) {
var membername = "";
if (typeof memres[0].username == "undefined") {
membername = "#"+memres[0].name;
} else {
membername = memres[0].username;
}
result = result + " " + membername;
});
}
result = result + "\n";
}
});
The issue that makes it sync is the for inside.
basically i'm generating a document in the result variable where i check the groups and tell the members so the expected output is
Group1 = member, member
Group2 = member, member

Answer 1

I usually use a pattern like the one below for this type of problem. In a nutshell: get a list of things then call a function to handle the list; that function calls itself until the list is completed; gather up the results as you go in an accumulator; when the list is empty, return whatever you've accumulated through the callback. It's just another way of accomplishing what #Andrey Sidorov demonstrated in his response.
//cb is (err, res)
function getData(cb){
var querystr = "SELECT * FROM groups";
var result = "";
dbClient.query(querystr, function (err, res, fields) {
if (err)
cb(err);
else {
var groups = [];
for (var ndx in res)
groups = groups.concat(res[ndx].members.split(","));
getMembers(groups, [], cb);
}
});
}
function getMembers(members, sofar, cb){
var member = members.shift();
if (!member)
cb(null, sofar);
else {
var memberquery;
var params;
if (member.substr(0,1) == "#") {
memberquery = "SELECT name FROM groups WHERE id = ?";
params = [member.substr(1, member.length-1)];
} else {
memberquery = "SELECT username FROM users WHERE id = ?";
params = [member];
}
dbClient.query(memberquery, params, function(err, res) {
if (err)
cb(err);
else {
var membername = "";
if (typeof res[0].username == "undefined") {
membername = "#" + res[0].name;
} else {
membername = res[0].username;
}
sofar.push(membername);
getMembers(members, sofar, cb);
}
});
}
}

Answer 2

function do_queries( resultCallback )
{
var querystr = "SELECT * FROM groups";
var result = "";
var num_queries_left = 0;
dbClient.query(querystr, function (err, res, fields) {
if (err) {
console.log(err);
return;
}
// calculate number of invocations of second sub-query
for(var i in res)
num_queries_left += res[i].members.split(",").length;
for(var i in res) {
result = result + "#" +res[i].name + " =";
for (var j in res[i].members.split(",")) {
var memberquery;
if (j.substr(0,1) == "#") {
memberquery = "SELECT name FROM groups WHERE id = "+j.substr(1, j.length-1);
} else {
memberquery = "SELECT username FROM users WHERE id= "+j;
}
dbClient.query(memberquery, function(err, memres, fields) {
var membername = "";
if (typeof memres[0].username == "undefined") {
membername = "#"+memres[0].name;
} else {
membername = memres[0].username;
}
result = result + " " + membername;
num_queries_left--;
if (num_queries_left == 0)
{
resultCallback(result);
return;
}
});
}
result = result + "\n";
}
});
}
do_queries( function(result) {
console.log(result);
});