Multiply Varying number of Values Inside array: Mongodb - database

Question

I have arranged my data so that the documents belonging to the same customer id are aggregated into a single collection. The data format is as follows.
{
"items": [
{
"stock_code": [
"22617",
"22768",
"20749"
],
"description": [
"DESIGN",
"FAMILY PHOTO FRAME",
"ASSORTED CASES"
],
"quantity": [
18,
12,
84
],
"unit_price": [
4.95,
9.95,
6.35
]
}
],
"_id": 581485,
"customer_id": 17389,
"country": "United Kingdom"
}
I need to multiply the values of array quantity with corresponding unit_price and get a total for multiple documents in a new field. I have tried using the $reduce function and $map function to get the output but both of them result in "error"
Multiply only supports numeric types, and not arrays
Could you please suggest how should i go about accomplishing this.
Codes tried:
"$addFields": {"order_total" :
{
"$sum": {
"$map": {
"input": "$items",
"as": "items",
"in": { "$multiply": [
{ "$ifNull": [ "$$items.quantity", 0 ] },
{ "$ifNull": [ "$$items.unit_price", 0 ] }
]}
}
}
}
}
Second:
"order_total" : {
"$reduce" : {
"input" : "$items",
"initialValue" : Decimal128("0.00"),
"in": {
"$sum" : [
"$$value",
{"$multiply" : [ "$$this.quantity", "$$this.unit_price" ] }
]}
}
}
The expected result needs to add a new field of "total" by multiplying the corresponding entries of unit_price with quantity. The error message is that of multiply only supports numeric types and not arrays.

Answer 1

I would decompose the problem into solvable chunks beginning with the smallest unit and start with the two arrays quantity and unit_price.
So given just a document with the structure
{
"quantity": [ 18, 12, 84 ],
"unit_price": [ 4.95, 9.95, 6.35 ]
}
We can add another field with the totals for each element in both arrays i.e.
{
"quantity": [ 18, 12, 84 ],
"unit_price": [ 4.95, 9.95, 6.35 ],
"total": [ 89.1, 119.4, 533.4 ]
}
This field can be computed using $range within $map as
{
"total": {
"$map": {
"input": { "$range": [ 0, { "$size": "$quantity" } ] },
"as": "idx",
"in": {
"$let": {
"vars": {
"qty": { "$arrayElemAt": [ "$quantity", "$$idx" ] },
"price": {
"$ifNull": [
{ "$arrayElemAt": [ "$unit_price", "$$idx" ] },
0
]
}
},
"in": { "$multiply": [ "$$qty", "$$price" ] }
}
}
}
}
}
This then becomes basis for calculating the order_total field with two pipeline stages for clarity (although can be composed into a single pipeline with $reduce for brevity)
var totalMapExpression = {
"$map": {
"input": { "$range": [ 0, { "$size": "$$item.quantity" } ] },
"as": "idx",
"in": {
"$let": {
"vars": {
"qty": { "$arrayElemAt": [ "$$item.quantity", "$$idx" ] },
"price": {
"$ifNull": [
{ "$arrayElemAt": [ "$$item.unit_price", "$$idx" ] },
0
]
}
},
"in": { "$multiply": [ "$$qty", "$$price" ] }
}
}
}
};
db.collection.aggregate([
{ "$addFields": {
"items": {
"$map": {
"input": "$items",
"as": "item",
"in": {
"quantity": "$$item.quantity",
"unit_price": "$$item.unit_price",
"stock_code": "$$item.stock_code",
"description": "$$item.description",
"total": { "$sum": totalMapExpression }
}
}
}
} },
{ "$addFields": {
"order_total": { "$sum": "$items.total" }
} }
])

Answer 2

Try the below query:
db.collection.aggregate(
[{ $unwind: { path: "$items",} },
{ $unwind: { path: "$items.quantity",} },
{ $unwind: { path: "$items.unit_price",} },
{ $addFields: { 'total': {$multiply: ["$items.quantity", "$items.unit_price"] }} }])