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MongoDB run loop to add incremental index value in array

I have a MongoDB collection where I have an array of objects and I want to add an incremental index value to each element of the array of objects.

Like index starts from 0 to array length – 1.

db.collection.aggregate([
    {
        "$unwind": "$array"
    },
    {
        "$addFields": {
            "array.index": {
                "$add": [
                    {
                        "$ifNull": ["$index", 0]
                    },
                    1
                ]
            }
        }
    },
    {
        "$group": {
            "_id": "$_id",
            "array": {
                "$push": "$$ROOT.array"
            }
        }
    }
])
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2

Answers


  1. 0

    MongoDB playground link: https://mongoplayground.net/p/SWw7rIYHNcj

    db.collection.aggregate([
  {
    "$unwind": {
      "path": "$array",
      "includeArrayIndex": "index"
    }
  },
  {
    "$addFields": {
      "array.index": {
        "$toInt": "$index"
      }
    }
  },
  {
    "$group": {
      "_id": "$_id",
      "array": {
        "$push": "$$ROOT.array"
      }
    }
  }
])

    $unwind stage:
    Opens an array field ($array) from the input documents.
    Creates a new document for each element in the array.
    Includes the array index of each element in a new field using includeArrayIndex.

    $addFields stage:
    Adds a new field ("array.index") to each document.
    Converts the "index" field to an integer using the $toInt operator..

    $group stage:
    Groups documents based on the "_id" field.
    Creates an "array" field within each group using the $push operator.

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  2. 0

    Here’s another way to do it. Comments are in the pipeline.

    db.collection.aggregate([
  { // just add index if type is array
    "$match": {
      "array": {"$type": "array"}
    }
  },
  { // map each element with incremental index
    "$set": {
      "array": {
        "$map": {
          "input": {
            // produce index values
            "$range": [0, {"$size": "$array"}]
          },
          "as": "idx",
          "in": {
            // merge index into each object
            "$mergeObjects": [
              {"index": "$$idx"},
              {"$arrayElemAt": ["$array", "$$idx"]}
            ]
          }
        }
      }
    }
  }
])

    Try it on mongoplayground.net.

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