Getting the document field names and adding them as nested fields in an Aggregation

Question

Im writing a mongodb aggregate pipeline. My documents are of the form below :

   {
  name: "01"
  tags {
  tag_01: {
    a: "a01-val",
    b: "b01-val"
  },
  tag_02: {
    a: "a02-val",
    b: "b02-val"
  }
  }
},
{
  name: "02"
  tags {
  tag_03: {
    a: "a03-val",
    b: "b03-val"
  },
  tag_04: {
    a: "a04-val",
    b: "b04-val"
  }
  }
}

My pipeline should generate documents as below :

{
  name: "01"
  tags {
  tag_01: {
    a: "a01-val",
    b: "b01-val",
    tagName: "tag_01"
  },
  tag_02: {
    a: "a02-val",
    b: "b02-val",
    tagName: "tag_02"
  }
  }
},
{
  name: "02"
  tags {
  tag_03: {
    a: "a03-val",
    b: "b03-val",
    tagName: "tag_03"
  },
  tag_04: {
    a: "a04-val",
    b: "b04-val",
    tagName: "tag_04"
  }
  }
}

In other words, a new field is added inside each subdocument whose value is the name of the parent field. Is it possible to do this?

Answer 1

Use $objectToArray to convert tags into an array of k-v tuples. Use $map to iterate through the array elements and $mergeObjects to set the field with k. Then use $arrayToObject to revert back to original object form.

db.collection.update({},
[
  {
    "$set": {
      "tags": {
        "$objectToArray": "$tags"
      }
    }
  },
  {
    "$set": {
      "tags": {
        "$map": {
          "input": "$tags",
          "as": "tag",
          "in": {
            k: "$$tag.k",
            v: {
              "$mergeObjects": [
                "$$tag.v",
                {
                  "tagName": "$$tag.k"
                }
              ]
            }
          }
        }
      }
    }
  },
  {
    $set: {
      tags: {
        "$arrayToObject": "$tags"
      }
    }
  }
],
{
  multi: true
})

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