how to project by a value as a property in mongodb?-CodePudding

I have this Object:

{
    "_id":"1",
    "a":"b",
    "b":"wanted value"
},
{
    "_id":"2",
    "a":"c",
    "c":"wanted value 2"
}

how can i get the value of a and make it the property with which i project to get "wanted value"/"wanted value 2" ?

wanted output:

{
    "_id":"1",
    "b":"wanted value"
},
{
    "_id":"2",
    "c":"wanted value 2"
}

CodePudding user response：

Use $objectToArray to convert the root document into an array of k-v tuples then use $reduce to process the array.

db.collection.aggregate([
  {
    "$addFields": {
      "arr": {
        "$objectToArray": "$$ROOT"
      }
    }
  },
  {
    "$addFields": {
      // store value of field "a" into "v"
      "v": {
        "$reduce": {
          "input": "$arr",
          "initialValue": null,
          "in": {
            "$cond": {
              "if": {
                $eq: [
                  "$$this.k",
                  "a"
                ]
              },
              "then": "$$this.v",
              "else": "$$value"
            }
          }
        }
      }
    }
  },
  {
    "$addFields": {
      "result": {
        "$reduce": {
          "input": "$arr",
          "initialValue": null,
          "in": {
            "$cond": {
              "if": {
                // find from k-v tuples where the field name is same as value in previously deduced "v" field
                $eq: [
                  "$$this.k",
                  "$v"
                ]
              },
              "then": "$$this.v",
              "else": "$$value"
            }
          }
        }
      }
    }
  },
  {
    "$project": {
      // some cosmetics
      arr: false,
      v: false
    }
  }
])

Here is the Mongo playground for your reference.