Mongodb: how to sum at breaks in 2 fields? - mongodb

Question

I have a document sample as follows:
{
_id: ObjectId("5dfa4f7b9254a519a5c7f166"),
Date: "12/11/19",
Description: "Amazon",
Amount: 32.01,
DebitCredit: "debit",
Category: "Shopping",
Month: 12,
Day: 11,
Year: 2019,
Account_Name: "Gold Delta SkyMiles"
}...
This is my query:
db.checks.aggregate([
{ $match: { Category: "Shopping" } },
{ $project: { Year: 1, Month: 1, Category: 1, Amount: { $sum: "$Amount" } } },
{
$group: {
_id: {
Year: "$Year",
Month: "$Month",
Category: "$Category",
Amount: { $sum: "$Amount" }
}
}
},
{ $sort: { Year: 1, Month: 1 } }
]);
I'm looking for a total by each year / month combination. How can I achieve this?

Answer 1

You're almost there, you just need to tweak your $group stage a little:
From the docs:
Groups input documents by the specified _id expression and for each distinct grouping
So all we have to do is take out the amount field outside of the _id field.
{
$group: {
_id: {
Year: "$Year",
Month: "$Month",
},
Amount: { $sum: "$Amount" }
}
}
I removed category as it was redundant but feel free to add it back to your query.
EDIT:
For the $sort stage the fields Year & Month do not exists after the $group which is the reason its failing.
After the $group stage your documents are of the form:
{
_id: {Year: number, Month: number},
Amount: number
}
So just change your $sort into:
{ $sort: { '_id.Year': 1, '_id.Month': 1 } }