Move field from an json object to another object with condition

Question

I have the following JSON structure :

{
  "1": {
    "precedingId": "0",
    "value": "A"
  },
  "2": {
    "precedingId": "1",
    "value": "B"
  },
  "3": {
    "precedingId": "2",
    "value": "C"
  }
}

I'm trying to get the following output using Jolt :

{
  "1": {
    "precedingId": "0",
    "precedingValue" : null, // null because there is no object with key equals to 0
    "value": "A"
  },
  "2": {
    "precedingId": "1",
    "precedingValue" : "A",
    "value": "B"
  },
  "3": {
    "precedingId": "2",
    "precedingValue" : "B"
    "value": "C"
  }
}

For each object, take precedingId as a key and lookup 1 level above if there is any corresponding object and if so then take value and put it in current object as precedingValue.

I tried several specs for few days but I think it mights be not possible doing this kind of logic in Jolt ?

Answer 1

You can use the following transformation spec :

[
  {//group the attributes by inner "precedingId" values of each object
   //by @1,precedingId prefixes
   //except the precedingValues from the other objects    
   //in order to transfer them from their original object
   //by using &1 prefix 
    "operation": "shift",
    "spec": {
      "*": {
        "$": "@1,precedingId.id",
        "value": ["@1,precedingId.value", "&1.precedingValue"],
        "precedingId": "@1,precedingId.&"
      }
    }
  },
  {//this time group the attributes by the generated "id" values
    "operation": "shift",
    "spec": {
      "*": {
        "id": { "": "" }, //get rid of temporarily generated "id" values
        "*": "@1,id.&"
      }
    }
  },
  {//set the missing value
    "operation": "modify-overwrite-beta",
    "spec": {
      "*": {
        "~precedingValue": null// ~ operator means if this attribute does not exist or is null, then make it be null
      }
    }
  }
]

the demo on the site Jolt Transform Demo Using v0.1.1 is :