NodeJs with Mongoose - Nested queries async issue - node.js

Question

I have to mongodb collections, like this:
UserGroup collection
fields:
name: String
group_id: Number
User collection
fields:
user_name: String
group_id: Number
I want generate a report like this:
ADMINISTRATORS
--------------------------
jlopez
rdiaz
OPERATORS
--------------------------
amiralles
dcamponits
But I get the following report:
ADMINISTRATORS
--------------------------
OPERATORS
--------------------------
jlopez
rdiaz
amiralles
dcamponits
Following is the code to generate the report:
UserGroup.find({}, (err, groups) => {
for(var i in groups){
console.log(groups[i].name)
console.log("--------------------")
User.find( {group_id : groups[i].group_id}, (err, users) =>{
for(var j in users){
console.log(users[j].user_name)
}
})
}
})
Clearly, this is a problem of the NodeJs/Mongoose asynchronicity.
QUESTION: How do I make the first For cycle wait until the internal cycle ends for each UserGrop?
Thanks in advance,
David.

Answer 1

You can run an aggregation pipeline that uses $lookup to do a "left-join" to another collection in the same database to filter in documents from the "joined" collection for processing. With this you won't need any async library:
UserGroup.aggregate([
{
"$lookup": {
"from": "users",
"localField": "group_id",
"foreignField": "group_id",
"as": "users"
}
},
{
"$project": {
"_id": 0,
"name": 1,
"users": {
"$map": {
"input": "$users",
"as": "user",
"in": "$$user.user_name"
}
}
}
}
], (err, groups) => {
if (err) throw err;
console.log(JSON.stringify(groups, null, 4));
})
Sample Output
[
{
"name": "ADMINISTRATORS",
"users": ["jlopez", "rdiaz"]
},
{
"name": "OPERATORS",
"users": ["amiralles", "dcamponits"]
}
]

Answer 2

Add support for promises to mongoose. I use q, but you can use bluebird too.
mongoose.Promise = require('q').Promise;
Then you can use q.all to resolve once all of the user queries have completed.
var promises = [];
var groups = [];
UserGroup.find({}, (err, groups) => {
for(var i in groups){
groups.push(groups[i]);
promises.push(User.find( {group_id : groups[i].group_id}));
}
});
q.all(promises).then( function(usersByGroup){
var indx = 0;
usersByGroup.forEach(function(users){
var grp = groups[indx];
console.log(groups[i].name);
console.log("--------------------");
for(var j in users){
console.log(users[j].user_name)
}
indx++;
});
});

Answer 3

This is a good use case for asyc, you can get a get basic idea from following code. it is based on async each & waterfall. [ Please add proper error handling for the following code yourself.]
UserGroup.find({}, (err, groups) => {
async.each(groups, (group, callback) =>{
async.waterfall([
(wCallback) => {
User.find({group_id : group.group_id}, wCallback)
},
(users, wCallback) => {
console.log(group.name)
console.log("--------------------")
for(var j in users){
console.log(users[j].user_name)
}
wCallback()
}
], callback)
})
})