Node.js : control flow with forEach - arrays

Question

I am trying to create array from database objects :
I have entity "group" wich hasMany "devices", I want to create array whit all groups and for each groups the list of his devices :
[
{
"group_id": “1”,
"name": “My_group”,
"devices_list": [1, 2, 18]
},
{
"group_id": “2”,
"name": “My_second_group”,
"devices_list": [3, 24]
}
]
I tried several ways like this :
Group.all(function (err, groups) {
var resJson = {};
groups.forEach(function(group, index){
group.devices(function(err, devices){
resJson[index] = group;
console.log(devices);
resJson[index].devices_list = devices;
//End of the loop
if (index == groups.length -1){
send({code: 200, data: resJson});
}
});
});
});
EDIT 1 :
I tried this way too :
var resJson = {};
groups.forEach(function(group, index){
group.devices(function(err, devices){
resJson[index] = group;
resJson[index].devices_list = [];
devices.forEach(function(device,index2){
resJson[index].devices_list.push(device);
});
//End of the loop
if (index == groups.length -1){
send({code: 200, data: resJson});
}
});
});
But finally, my resJson contains only empty groups (groups without device associated), the other groups are not visible. Thus my devices_list are all empty whereas the console.log(devices) display devices.
It seems that the "send" instruction is processed before the treatment of non-empty groups.
What is the rigth way to do this ?
Thank you for your time

Answer 1

Instead of tracking and using an index against the length of the list perhaps you could use an after type of construct. I really enjoy them and they're easy to integrate and serve the perfect purpose for doing something after a set number of times.
First, lets define an after function you can use.
var after = function(amount, fn) {
var count = 0;
return function() {
count += 1;
if (count >= amount) {
fn.apply(arguments);
}
};
};
That should work for you now, let's modify your code sample to use this.
var json = []; // To return, you originally wanted an array.
Group.all(function(err, groups) {
if (err) {
// Handle the error
} else {
var sendJson = after(groups.length, function(json) {
send({code: 200, data: json});
});
groups.forEach(function(group) {
group.devices(function(err, devices) {
if (err) {
// Handle the error...
} else {
group.devices_list = devices;
json.push(group); // This part is different, using this method you'll match the JSON you gave as your "goal"
}
// This is outside the if/else because it needs to be called for every group
// regardless of change. If you do not call this the exact number of times
// that you specified it will never fire.
sendJson(json);
});
}
});
Perhaps something like that might clear up your issue.