Update all array subobjects relative to their own current value-CodePudding

Let's say I want to multiply all nested values by 10.

{
  foos: [
    { val: 1 },
    { val: 10 },
    { val: 5 },
  ]
}
// to
{
  foos: [
    { val: 10 },
    { val: 100 },
    { val: 50 },
  ]
}

Since we reuse the existing field values, I assume I will have to use aggregation operators.

Here some attempts:

// Does not work, since it it unwinds the values from their parent object.
coll.updateMany({}, [
  {$set: {
    foos: {$map: {
      input: '$foos',
      as: 'foo',
      in: {$multiply: ['$$foo.val', 10]}
    }}
  }}
])

// MongoServerError: $multiply only supports numeric types, not string
coll.updateMany({}, [
  {
    $set: {
      'foos.val': {$multiply: ['$foos.$.val', 10]}
    }
  }
])

Is this possible without fetching the doc, and proceeding to the update with a JS loop?

Something like:

coll.updateMany({}, [
  {
    $set: {
      foos: {
        $map: {
          input: '$foos',
          as: 'foo',
          in: {
            $project: {
              blackMagicToSpreadtheOriginalFoo: '$$foo', // FIXME
              val: { $multiply: ['$$foo.val', 10] }
            }
          }
        }
      }
    }
  }
])

CodePudding user response：

In this case, why do you need: blackMagicToSpreadtheOriginalFoo: '$$foo', // FIXME? Why not simply:

coll.updateMany({},
[
  {
    $set: {
      foos: {
        $map: {
          input: "$foos",
          in: {val: {$multiply: ["$$this.val", 10]}}
        }
      }
    }
  }
])

See how it works on the playground example

In case you have other keys inside etch item, just use $mergeObjects:

coll.updateMany({},
[
  {
    $set: {
      foos: {
        $map: {
          input: "$foos",
          in: {
            $mergeObjects: [
              "$$this",
              {val: {$multiply: ["$$this.val", 10]}}
            ]
          }
        }
      }
    }
  }
])

See how it works on the playground example - with keys