mongo Aggregate and return results by months - mongodb

Question

I have this...
$group: {
'_id': this.userId,
'hours': {
$sum: '$hours'
},
'magazines': {
$sum: '$magazines'
},
'brochures': {
$sum: '$brochures'
},
'books': {
$sum: 'books'
}
}
}, {
$project: {
hours: '$hours',
magazines: '$magazines',
brochures: '$brochures',
books: '$books'
}
... which returns the sum of the above fields.
What I want to do is, return the total sum of, say, hours for a particular user, grouped into months.
Something like:
January:
userId:
hours: 10
magazines: 10
....
February:
userId:
hours:2
magazines: 2
etc

Answer 1

Use the following aggregation pipeline which uses the date aggregation operators $year and $month to get the total sums for the given attributes per user grouped into months:
var pipeline = [
{
"$group": {
"_id": {
"userId": "$userId"
"year": { "$year": "$date" }
"month" { "$month": "$date" }
},
'hours': {
$sum: '$hours'
},
'magazines': {
$sum: '$magazines'
},
'brochures': {
$sum: '$brochures'
},
'books': {
$sum: 'books'
}
]
}
]
db.collection.aggregate(pipeline);
To get the exact schema where you convert the month and userId values to keys, use the map() method from the cursor returned by the aggregate() method:
var result = db.collection.aggregate(pipeline).map(function (doc){
var obj = {};
obj[doc._id.month][doc._id.userId]["hours"] = doc.hours;
obj[doc._id.month][doc._id.userId]["magazines"] = doc.magazines;
obj[doc._id.month][doc._id.userId]["brochures"] = doc.brochures;
obj[doc._id.month][doc._id.userId]["books"] = doc.books;
return obj;
});
printjson(result);

Answer 2

This is the solution that works. Answer on MongoDB-User group:
[{
$group: {
_id: {
$month: "$createdAt"
},
hours: {
$sum: "$hours",
},
magazines: {
$sum: "$magazines"
},
brochures: {
$sum: "$brochures"
},
books: {
$sum: "$books"
}
}
}]