MongoDB: get sum by year/month with nested values - mongodb

Question

I'm trying to sum (spending by month/year) of a collection with nested amounts - with no luck.
This is the collection (extract):
[
{
"_id" : ObjectId("5faaf88d0657287993e541a5"),
"segment" : {
"l1" : "Segment A",
"l2" : "001"
},
"invoiceNo" : "2020.10283940",
"invoicePos" : 3,
"date" : ISODate("2019-09-06T00:00:00.000Z"),
"amount" : {
"document" : {
"amount" : NumberDecimal("125.000000000000"),
"currCode" : "USD"
},
"local" : {
"amount" : NumberDecimal("123.800000000000"),
"currCode" : "CHF"
},
"global" : {
"amount" : NumberDecimal("123.800000000000"),
"currCode" : "CHF"
}
}
},
...
]
I would like to sum up the aggregated invoice volume per month in "global" currency.
I tried this query on MongoDB:
db.invoices.aggregate(
{$project : {
month : {$month : "$date"},
year : {$year : "$date"},
amount : 1
}},
{$unwind: '$amount'},
{$group : {
_id : {month : "$month" ,year : "$year" },
total : {$sum : "$amount.global.amount"}
}})
I am getting as result this:
/* 1 */
{
"_id" : ObjectId("5faaf88d0657287993e541a5"),
"amount" : {
"document" : {
"amount" : NumberDecimal("125.000000000000"),
"currCode" : "USD"
},
"local" : {
"amount" : NumberDecimal("123.800000000000"),
"currCode" : "CHF"
},
"global" : {
"amount" : NumberDecimal("123.800000000000"),
"currCode" : "CHF"
}
},
"month" : 9,
"year" : 2019
}
/* 2 */
{
"_id" : ObjectId("5faaf88d0657287993e541ac"),
"amount" : {
"document" : {
"amount" : NumberDecimal("105.560000000000"),
"currCode" : "CHF"
},
"local" : {
"amount" : NumberDecimal("105.560000000000"),
"currCode" : "CHF"
},
"global" : {
"amount" : NumberDecimal("105.560000000000"),
"currCode" : "CHF"
}
},
"month" : 11,
"year" : 2020
}
This however does not sum up all invoices per month, but looks like single invoice lines - no aggregation.
I would like to get a result like this:
[
{
"month": 11,
"year": 2020,
"amount" : NumberDecimal("99999.99")
},
{
"month": 10,
"year": 2020,
"amount" : NumberDecimal("99999.99")
},
{
"month": 9,
"year": 2020,
"amount" : NumberDecimal("99999.99")
}
]
What is wrong with my query?

Answer 1

Would this be helpful?
db.invoices.aggregate([
{
$group: {
_id: {
month: {
$month: "$date"
},
year: {
$year: "$date"
}
},
total: {
$sum: "$amount.global.amount"
}
}
},
{$sort:{"_id.year":-1, "_id.month":-1}}
])
Playground
If you need any extra explanation let me know, but the code is pretty short and self-explanatory.

Answer 2

In principle your aggregation pipeline is fine, there a few mistakes:
An aggregation pipeline expects an array
$unwind is useless, because $amount is not an array. One element in -> one document out
You can use date function directly
So, short and simple:
db.invoices.aggregate([
{
$group: {
_id: { month: { $month: "$date" }, year: { $year: "$date" } },
total: { $sum: "$amount.global.amount" }
}
}
])