In MondoDB : Slice array of objects from slicing index array and push in array through aggregation

Question

I want to slice array in document according to slicingIndex key available in the same document and push into an array.

[
  {
    slicingIndex: [0, 4, 7, 10],
    array: [
      { name: 1 },
      { name: 2 },
      { name: 3 },
      { name: 4 },
      { name: 5 },
      { name: 6 },
      { name: 7 },
      { name: 8 },
      { name: 9 },
      { name: 10 }
    ]
  }
]

Output (expected):

{
  "array": [
    [
      { "name": 1 },
      { "name": 2 },
      { "name": 3 },
      { "name": 4 }
    ],
    [
      { "name": 5 },
      { "name": 6 },
      { "name": 7 }
    ],
    [
      { "name": 8 },
      { "name": 9 },
      { "name": 10 }
    ]
  ]
}

This is what aggregation i tried below but not working as expected:

db.collection.aggregate([
  {
    "$project": {
      slicingIndex: 1,
      array: {
        "$map": {
          "input": "$slicingIndex",
          "as": "num",
          "in": {
            "$slice": [
              "$array",
              "$$num",
              {
                "$subtract": [
                  {
                    "$arrayElemAt": ["$slicingIndex", { "$indexOfArray": ["$slicingIndex", "$$num"] }]
                  },
                  "$$num"
                ]
              }
            ]
          }
        }
      }
    }
  }
])

Answer 1

MongoDb Playground Link : https://mongoplayground.net/p/oNzBVLRfIU0

$add & $indexOfArray used to take one step ahead element from current pointer in array like (n+1).
$let : I have used it as optional to make variable reusable if needed.
$map : To traverse array elements.
$arrayElemAt : To get from element from array.
$subtract : To perform operation like [ (n+1) - n ] where n is index in array.
$filter : To filter out elements (in our case: null).

db.collection.aggregate([
  {
    "$project": {
      array: {
        "$map": {
          "input": "$slicingIndex",
          "as": "elem",
          "in": {
            "$let": {
              "vars": {
                //create variable that holds element one index+1 to the current index
                "elemOneStepHeadToElem": {
                  "$arrayElemAt": [
                    "$slicingIndex",
                    {
                      $add: [
                        1,
                        {
                          "$indexOfArray": [
                            "$slicingIndex",
                            "$$elem"
                          ]
                        }
                      ]
                    }
                  ]
                }
              },
              "in": {
                "$slice": [
                  "$array",
                  "$$elem",
                  {
                    "$subtract": [
                      "$$elemOneStepHeadToElem",
                      "$$elem"
                    ]
                  }
                ]
              }
            }
          }
        }
      }
    }
  }//remove null from array from last index
  ,
  {
    "$project": {
      array: {
        "$filter": {
          "input": "$array",
          "cond": {
            $ne: [
              "$$this",
              null
            ]
          }
        }
      }
    }
  }
])