Nodejs + Mongodb: find data after aggregation - node.js

Question

I'm a new to Nodejs and MongoDB.
Here is a sample of my dataset:
{
'name': ABC,
'age':24,
'gender':male,
...
}
Generally speaking, what I want to do is to aggregate data before using them to find different data clusters.
To be specific, I want to know how many people there are at different ages. Then, to find people(documents) at each age and store them.
Here is my code:
MongoClient.connect(url, function(err, db) {
if(err) {
console.log('Unable to connect to the mongoDB server. Error:', err);
} else {
db.collection('test').aggregate(
[
{ $group: { _id: "$age" , total: { $sum: 1 } } },
{ $sort: { total: -1 } }
]).toArray(function(err, result) {
assert.equal(err, null);
age = [];
for(var i in result) {
age.push(result[i]['_id'])
};
ageNodes = {};
for(var i in age) {
nodes = [];
var cursor = db.collection('test').find({'age':age[i]});
// query based on aggregated data
cursor.each(function(err,doc){
if(doc!=null){
nodes.push(doc);
} else {
console.log(age[i]);
ageNodes[age[i]] = nodes;
}
})
}
res.json(ageNodes);
});
};
});
My expected JSON format:
{
age:[different documents]
}
example:
{
20:[{name:A,gender:male,...},{},...],
30:[{name:B,gender:male,...},{},...],
...
}
However, what I got was an empty result, so I think maybe it was caused by the for loop.
I have no idea how to handle the asynchronous callback.

Answer 1

You only need to run the following pipeline which uses $push to add the root document (represented by $$ROOT system variable in the pipeline) to an array per age group:
Using MongoDB 3.4.4 and newer:
MongoClient.connect(url, function(err, db) {
if(err) {
console.log('Unable to connect to the mongoDB server. Error:', err);
} else {
db.collection('test').aggregate([
{ '$group': {
'_id': '$age',
'total': { '$sum': 1 },
'docs': { '$push': '$$ROOT' }
} },
{ '$sort': { 'total': -1 } },
{ '$group': {
'_id': null,
'data': {
'$push': {
'k': '$_id',
'v': '$docs'
}
}
} },
{ '$replaceRoot': {
'newRoot': { '$arrayToObject': '$data' }
} }
]).toArray(function(err, results) {
console.log(results);
res.json(results);
});
};
});
Using MongoDB 3.2 and below:
MongoClient.connect(url, function(err, db) {
if(err) {
console.log('Unable to connect to the mongoDB server. Error:', err);
} else {
db.collection('test').aggregate([
{ '$group': {
'_id': '$age',
'total': { '$sum': 1 },
'docs': { '$push': '$$ROOT' }
} },
{ '$sort': { 'total': -1 } }
]).toArray(function(err, results) {
console.log(results);
var ageNodes = results.reduce(function(obj, doc) {
obj[doc._id] = doc.docs
return obj;
}, {});
console.log(ageNodes);
res.json(ageNodes);
});
};
});