MongoDB 阵列内的 $project

Question

I have MongoDB collection items with following document:

{
    data: [{"name": "First"}, {"name": "Second"}],
    data2: [{"value": 20}, {"value": 30}]
}

Is there any way to project data2.value into data.value on same indexes? I would need this result:

{
    data: [
        {"name": "First", value: 20},
        {"name": "Second", value: 30}
    ]
}

It would like something like that:

db.items.aggregate([{"$project": {"data.X.value": "$data2.X.value", "data2": 0}}]) // Where X "iterate" over all indexes (so 0 and 1).

Lengths of arrays data and data2 are always same, but it doesn't have to be 2.

Answer 1

You can try,

• $map to loop through object by object of data array
• $indexOfArray to get the index of current object $$d
• $arrayElemAt to get the element value of data2 from particular $$d object index

db.collection.aggregate([
  {
    $project: {
      data: {
        $map: {
          input: "$data",
          as: "d",
          in: {
            name: "$$d.name",
            value: {
              $arrayElemAt: [
                "$data2.value",
                { $indexOfArray: ["$data", "$$d"] }
              ]
            }
          }
        }
      }
    }
  }
])

Playground: https://mongoplayground.net/p/wv1vEVsrHeT