Correct query for group by user, per month - mongodb

Question

I have MongoDB collection that stores documents in this format:
"name" : "Username",
"timeOfError" : ISODate("...")
I'm using this collection to keep track of who got an error and when it occurred.
What I want to do now is create a query that retrieves errors per user, per month or something similar. Something like this:
{
"result": [
{
"_id": "$name",
"errorsPerMonth": [
{
"month": "0",
"errorsThisMonth": 10
},
{
"month": "1",
"errorsThisMonth": 20
}
]
}
]
}
I have tried several different queries, but none have given the desired result. The closest result came from this query:
db.collection.aggregate(
[
{
$group:
{
_id: { $month: "$timeOfError"},
name: { $push: "$name" },
totalErrorsThisMonth: { $sum: 1 }
}
}
]
);
The problem here is that the $push just adds the username for each error. So I get an array with duplicate names.

Answer 1

You need to compound the _id value in $group:
db.collection.aggregate([
{ "$group": {
"_id": {
"name": "$name",
"month": { "$month": "$timeOfError" }
},
"totalErrors": { "$sum": 1 }
}}
])
The _id is essentially the "grouping key", so whatever elements you want to group by need to be a part of that.
If you want a different order then you can change the grouping key precedence:
db.collection.aggregate([
{ "$group": {
"_id": {
"month": { "$month": "$timeOfError" },
"name": "$name"
},
"totalErrors": { "$sum": 1 }
}}
])
Or if you even wanted to or had other conditions in your pipeline with different fields, just add a $sort pipeline stage at the end:
db.collection.aggregate([
{ "$group": {
"_id": {
"month": { "$month": "$timeOfError" },
"name": "$name"
},
"totalErrors": { "$sum": 1 }
}},
{ "$sort": { "_id.name": 1, "_id.month": 1 } }
])
Where you can essentially $sort on whatever you want.