MongoDb $addFields inside arrays that $multiply two values within the array
aggregation-framework, mongodb
Solution
Modify your second attempt using the `$map` operator to map the whole snapshot embedded document with its fields as
db.mycoll.aggregate([
{
"$addFields": {
"snapshot": {
"$map": {
"input": "$snapshot",
"as": "row",
"in": {
"id": "$$row.id",
"course": "$$row.course",
"quantity": "$$row.quantity",
"total": { "$multiply": [
{ "$ifNull": [ "$$row.quantity", 0 ] },
{ "$ifNull": [ "$$row.course", 0 ] }
]}
}
}
}
}
}
])
Problem
MongoDb version 3.4.4 How to aggregate a new key 'total', whose value is the product of 'course' and 'quantity' for each object inside array snapshot. Sample document: ``` { cur: "EUR", snapshot: [ { id: "24352345", course: 58.12, quantity: 13 }, { id: "34552345", course: 18.12, quantity: 63 } ] } ``` Desired result: ``` { cur: "EUR", snapshot: [ { id: "24352345", course: 58.12, quantity: 13, total: 755.56 }, { id: "34552345", course: 18.12, quantity: 63, total: 1141.56 } ] } ``` first attempt: ``` db.mycoll.aggregate([{ $addFields: { "snapshot.total": { $multiply:["$snapshot.quantity", "$snapshot.course"] } } }]) ``` "errmsg" : "$multiply only supports numeric types, not array" Second attempt: ``` db.mycoll.aggregate( [ { "$addFields": { "snapshot.total": { "$map": { "input": "$snapshot", "as": "row", "in": { "$multiply": [ { "$ifNull": [ "$$row.quantity", 0 ] }, { "$ifNull": [ "$$row.course", 0 ] } ]} } } }} ]) ``` The undesired value of 'total' is an array with the totals of all the objects: ``` { cur: "EUR", snapshot: [ { id: "24352345", course: 58.12, quantity: 13, total: [ 755.56, 1141.56 ] }, { id: "34552345", course: 18.12, quantity: 63, total: [ 755.56, 1141.56 ] } ] } ```