Aggregating Mongo collection by year and then by month - mongodb

Question

I have a Mongo collection that looks like this with a bunch of months, days, years:
[
{
"Date": ISODate("2021-08-05T04:59:54.000Z"),
"Amount": 999,
"Business": "Business 1",
},
{
"Date": ISODate("2021-08-05T04:59:54.000Z"),
"Amount": 5.99,
"Business": "Business 2",
},
{
"Date": ISODate("2021-07-17T21:41:56.000Z"),
"Amount": 20000,
"Business": "Business 2",
},
{
"Date": ISODate("2021-06-17T21:41:56.000Z"),
"Amount": 200,
"Business": "Business 5",
}
]
I have done an aggregation like this
db.collection.aggregate({
$group: {
_id: {
year: {
$year: "$Date"
},
month: {
$month: "$Date"
}
},
sum: {
$sum: "$Amount"
}
}
})
...which partially gives me what I want which is a sum of amounts per year and month.
[
{
"_id": {
"month": 6,
"year": 2021
},
"sum": 200
},
{
"_id": {
"month": 7,
"year": 2021
},
"sum": 20000
},
{
"_id": {
"month": 8,
"year": 2021
},
"sum": 1004.99
}
]
What I would like however is to have something like the below where the year is at the top and the months are aggregated in a sum so that it's easier to iterate in the frontend but I have not been able to get it no matter what I have tried:
[
{
"year": 2021,
"sumAmount": 21204.99,
"months": [
{
"month": 7,
"amount": 20000
},
{
"month": 6,
"amount": 200
},
{
"month": 8,
"amount": 1004.99
}
]
},
{ "year" : 2020,
....
}
]
I have been pretty close in using another $group and $push but I have not been able to get what in my mind is a second group by month. Any help will be appreciated!

Answer 1

You just need one more $group to get your expected result. For another sorting, you can put an $sort after the $group stage. You will need to use $push to keep the ordering in the final array.
db.collection.aggregate([
{
$group: {
_id: {
year: {
$year: "$Date"
},
month: {
$month: "$Date"
}
},
sum: {
$sum: "$Amount"
}
}
},
{
"$sort": {
"_id.year": 1,
"_id.month": 1
}
},
{
"$group": {
"_id": "$_id.year",
"sumAmount": {
$sum: "$sum"
},
"months": {
"$push": {
"month": "$_id.month",
"amount": "$sum"
}
}
}
}
])
Here is the Mongo playground for your reference.