如何使用聚合将2个数组的对应元素相乘？

Question

I want to multiply the individual elements of 2 arrays:

{
  "lessonHours": [2, 1, 2]
},
{
  "studentCount": [2, 6, 5]
}

I have tried $multiply but since it works only for numeric values and not array.

I need something like this:

{
  "product": [4, 6, 10]
}

First element is 2(1st array)*2(2nd array) = 4. Then 1*6, 2*5

Answer 1

Use .map . Don't use .forEach because you'll have to manually push the value back.

let calculatedValues = lessonHours.map((item, index) => item * studentCount[index])

This is only applicable if both your arrays have the same length. You'll need to restructure your code if both arrays won't have the same length.

Answer 2

You can use below aggregation

db.collection.aggregate([
  { "$project": {
    "product": {
      "$map": {
        "input": { "$range": [ 0, { "$size": "$lessonHours" }] },
        "in": {
          "$multiply": [
            { "$arrayElemAt": ["$lessonHours", "$$this"] },
            { "$arrayElemAt": ["$studentCount", "$$this"] }
          ]
        }
      }
    }
  }}
])

Output

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "product": [
      4,
      6,
      10
    ]
  }
]