Calculated fields in the same $project stage of the pipeline aggregate mongodb?

Question

I am new working with mongodb. I am trying to use calculated fields in the $project stage, to resolve other fields.

I show you a simplified example.

input

    [
  {
    a: 5,
    b: 3
  },
  {
    a: 2,
    b: 1
  },
  
]

Code

db.collection.aggregate([
  {
    $project: {
      _id: 0,
      c: {
        "$add": [
          "$a",
          "$b"
        ]
      },
      d: {
        "$multiply": [
          "$a",
          "$c"
        ]
      }
    }
  }
])

Output

[
  {
    "c": 8,
    "d": null
  },
  {
    "c": 3,
    "d": null
  }
]

result

I only get null values, I have tried to solve it using $let without results. A simple way to solve it would be to replicate the $add operation, but if it is a complex calculation like the one I am dealing with in my real project and it is replicated many times, as is the case, it could be performing unnecessary operations.

Help to me please

sample playgrond

Answer 1

db.collection.aggregate([
{
"$project": {
  "test": {
    "$let": {
      "vars": {
        "sum": {
          $add: [
            "$a",
            "$b"
          ]
        },
        "d": 3
      },
      "in": {
        "c": "$$sum",
        "d": {
          "$multiply": [
            "$$sum",
            "$b"
          ]
        }
      }
     }
    }
   }
  }
])

Explained:

Set the sum via $let and use it later as variable to calculate the multiplication.

Playground