Group items inside of another group - mongodb

Question

I want to group the following collection by category and sum its total value, then create a subcategory attribute, on same structure, summing subcategories if more than one equal.
[
{
"_id": 1,
"date": ISODate("2019-10-10T00:00:00.000Z"),
"description": "INTERNET BILL",
"credit": "",
"debit": "-100.00",
"category": "home",
"subcategory": "internet",
"__v": 0
},
{
"_id": 2,
"date": ISODate("2019-10-10T00:00:00.000Z"),
"description": "WATER BILL",
"credit": "",
"debit": "-150.00",
"category": "home",
"subcategory": "water",
"__v": 0
},
{
"_id": 3,
"date": ISODate("2019-10-10T00:00:00.000Z"),
"description": "MC DONALDS",
"credit": "",
"debit": "-30.00",
"category": "food",
"subcategory": "restaurants",
"__v": 0
},
{
"_id": 4,
"date": ISODate("2019-10-10T00:00:00.000Z"),
"description": "BURGER KING",
"credit": "",
"debit": "-50.00",
"category": "food",
"subcategory": "restaurants",
"__v": 0
},
{
"_id": 5,
"date": ISODate("2019-10-10T00:00:00.000Z"),
"description": "WALMART",
"credit": "",
"debit": "-20.00",
"category": "food",
"subcategory": "groceries",
"__v": 0
},
]
Desireble output:
[
{
"_id": "home",
"total": "-250.00",
"subcategory" : [
{"id": "internet", "total": "-100"},
{"id": "water", "total": "-150"}
]
},
{
"_id": "food",
"total": "-100.00",
"subcategory" : [
{"id": "restaurants", "total": "-80"},
{"id": "groceries", "total": "-20"}
]
}
]
With the following query, I've almost achieved it, but I haven't find a way to sum values on subcategories.
db.getCollection('expenses').aggregate([
{$match:
{"date" : { "$gte" : new Date("11-10-2019"), "$lte": new Date("2019-10-11") }}
},
{$group: {
_id: "$category",
total: { $sum: { $toDouble: { $cond: { if: { $ne: [ "$debit", ""] }, then: "$debit", else: "$credit" } } } },
subcategories: { $addToSet: {id: "$subcategory" }},
}}
])

Answer 1

You can to $group twice (by subcategory first):
db.collection.aggregate([
{
$group: {
_id: { category: "$category", subcategory: "$subcategory" },
total: { $sum: { $toDouble: "$debit" } }
}
},
{
$group: {
_id: "$_id.category",
total: { $sum: "$total" },
subcategories: { $push: { id: "$_id.subcategory", total: "$total" } }
}
}
])
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