Aggregating a total from subdocuments in MongoDB - arrays

Question

I have a document like the one below, I'd essentially like to produce an aggregate for the items in a sub document.
Essentially each document is a sales record, which has details of the sales and a sub document / array with the qtys of each item sold.
I'd like to produce a summary of all the items sold.
So an example collection is:
{
non_relevant_1: "ABC",
non_relevant_2: "DEF",
items_array: {
"item_1": 1,
"item_2": 2,
"item_3": 1,
"item_4": 1
}
},
{
non_relevant_1: "HIJ",
non_relevant_2: "KLM",
items_array: {
"item_1": 3,
"item_2": 2,
"item_3": 4
}
}
I'd then like to be able to produce something like:
{
items_array: {
"item_1": 4,
"item_2": 4,
"item_3": 5,
"item_4": 1
}
}
Many thanks in advance.

Answer 1

I think you need to change your schema, you are saving data in keys.
MongoDB operators are not made to have unknown keys, for example we can't group by an unknown key.To do those we do complicated and slow things like $objectToArray.
Also the data that you want as results have the same problem.
If you look at the query only the middle $unwind and $group would be needed it, with a changed schema, and asking for data without data in keys.
I mean instead of
items_array: {
"item_1": 1,
"item_2": 2,
"item_3": 1,
"item_4": 1
}
Your collection should have being like(first part of the query does that changing your schema)
items_array: [
{"name" "item_1",
"qty" : 1},
{"name" "item_2",
"qty" : 2},
{"name" "item_3",
"qty" : 1},
{"name" "item_4",
"qty" : 1}
]
Also the results should have known keys only.
Maybe the reason you were stuck is that.You will make things much easier for you.
Test code here
Query (query works, for your schema but i told you what i think)
db.collection.aggregate([
{
"$addFields": {
"items_array": {
"$map": {
"input": {
"$map": {
"input": {
"$objectToArray": "$items_array"
},
"as": "m",
"in": [
"$$m.k",
"$$m.v"
]
}
},
"as": "item",
"in": {
"name": {
"$arrayElemAt": [
"$$item",
0
]
},
"qty": {
"$arrayElemAt": [
"$$item",
1
]
}
}
}
}
}
},
{
"$unwind": {
"path": "$items_array"
}
},
{
"$group": {
"_id": "$items_array.name",
"total-qty": {
"$sum": "$items_array.qty"
}
}
},
{
"$group": {
"_id": null,
"items_array": {
"$push": {
"$map": {
"input": {
"$map": {
"input": {
"$objectToArray": "$$ROOT"
},
"as": "m",
"in": [
"$$m.k",
"$$m.v"
]
}
},
"as": "i",
"in": {
"$arrayElemAt": [
"$$i",
1
]
}
}
}
}
}
},
{
"$project": {
"_id": 0
}
},
{
"$addFields": {
"items_array": {
"$arrayToObject": "$items_array"
}
}
}
])