Group by months and get their counts in MongoDB - mongodb

Question

I have a document with the following structure.
{
"_id" : ObjectId(""),
"review_id" : "1",
"product_id" : "1",
"date" : 1638869377,
"rating" : 5,
"title" : "lorem",
"review" : "lorem",
"updated_at" : ISODate("2021-12-07T07:10:55.732Z"),
"created_at" : ISODate("2021-12-07T05:04:11.750Z")
}
I managed to get the number of user comments by month. But I want to get it separately for each rating.
For example, in the 10th month there are 7 comments and how can I get 2 of them 1 star, 3 of them 4 stars, 2 of them 5 stars?
{
"$group": {
"_id": {
"month": {
"$month": {
"$toDate": "$date"
}
}
},
"reviews": {
"$sum": 1
}
}
}

Answer 1

You could add everything you want to group by inside the _id field. Just adding a new field with the rating would do the job.
{
"$group": {
"_id": {
"month": {
"$month": {
"$toDate": "$date"
}
},
"rating": "$rating"
},
"reviews": {
"$sum": 1
}
}
}
Note: I would also add a group by year, so you can get each month separed properly. If you dont add them, Mongo will group every January of every year together for example.
Playground

Answer 2

You just want to $group by both fields, the month and the rating, like so:
db.collection.aggregate([
{
"$group": {
"_id": {
"month": {
"$month": {
"$toDate": "$date"
}
},
rating: "$rating"
},
"reviews": {
"$sum": 1
}
}
}
])
Mongo Playground
Or my recommendation just to add a little more structuring to the output is to use this pipeline.