How to get the Sum from independent two subdocument Arrays in mongoose - node.js

Question

I have this schema
var salesExpenseSchema = new Schema({
date : {
month: Number
},
sales: [{amount : Schema.Types.Decimal128}],
expenses: [{amount : Schema.Types.Decimal128}]
});
Example of a database record is like this
{
_id:'5dbac5dfa1488240cbc4f838',
date:{month:11},
sales:[{amount:3000},{amount:5000}],
expenses: [{amount:5000},{amount:500}]
},
{
_id:'5dbac5dfa1488240cbc4f838',
date:{month:10},
sales:[{amount:2000},{amount:5000}],
expenses: [{amount:500},{amount:800}]
},
{
_id:'5dbac5dfa1488240cbc4f838',
date:{month:09},
sales:[{amount:2000},{amount:4000}],
expenses: [{amount:200},{amount:300}]
}
Now I want to get the Summation of sales and expenses.
I have used Aggregate with $unwind for both sales and expenses like this below:
SalesExpense.aggregate([
{$unwind: "$sales"},
{$unwind: "$expenses"},
{$group:{
_id:'$_id',
sales:{$sum: "$sales.sellPrices"},
expenses:{$sum: "$expenses.amount"},
}
},
But the problem is... If one array document has data and the other has no data, then it gives 0 ie, the real summation isn't obtained. This is to say, if there are sales but no expenses then their sum becomes 0, and vice-versa.
I want to get the summation for both sales and expenses regardless of one of them not having data. How do I achieve this?
EDIT:
I have edited the question and added the date object in my schema and in the database records: I want to make this summation based on each month, that is to say... each month to have its own sales and expenses... Sort of a timeline with each month having its own sales and expenses.
I have tried using $group before $project
{$group:{
_id:'$date.month'}}
But it seems not to give the expected results.
I want an output like this one:
[
{
"month": "11",
"sales": {
"$numberDecimal": "8000"
},
"expenses": {
"$numberDecimal": "5500"
}
},
{
"month": "10",
"sales": {
"$numberDecimal": "7000"
},
"expenses": {
"$numberDecimal": "1100"
}
},
{
"month": "09",
"sales": {
"$numberDecimal": "6000"
},
"expenses": {
"$numberDecimal": "500"
}
},
]
How can I achieve this?

Answer 1

You can group by month and get the totals like this:
db.collection.aggregate([
{
$group: {
_id: "$date.month",
"sales": {
"$sum": {
"$sum": "$sales.amount"
}
},
"expenses": {
"$sum": {
"$sum": "$expenses.amount"
}
}
}
}
])
Sample Data:
[
{
_id: "5dbac5dfa1488240cbc4f838",
date: {
month: 11
},
sales: [
{
amount: 1
},
{
amount: 2
}
],
expenses: []
},
{
_id: "5dbac5dfa1488240cbc4f839",
date: {
month: 11
},
sales: [
{
amount: 5
},
{
amount: 6
}
],
expenses: [
{
amount: 7
},
{
amount: 8
}
]
},
{
_id: "5dbac5dfa1488240cbc4f840",
date: {
month: 12
},
sales: [],
expenses: [
{
amount: 7
},
{
amount: 8
}
]
}
]
Result:
[
{
"_id": 12,
"expenses": 15,
"sales": 0
},
{
"_id": 11,
"expenses": 15,
"sales": 14
}
]
Playground:
https://mongoplayground.net/p/K9ofoZx5ORI