How to filter keys by the value of another elements with JOLT?

Question

I have the following input JSON:

{
  "2021" : [ "a", "b", "c" ],
  "2022" : [ "d", "e", "f" ],
  "2023" : [ "g", "h", "i" ],
  "2024" : [ "j", "k", "l" ],
  "2025" : [ "m", "n", "o" ],
  "year" : "2022"
}

And I need to filter the keys that are equal to the value of 'year'. In this case, I just need the key '2022'. Something like that:

{
  "results_this_year" : [ "d", "e", "f" ],
  "year" : "2022"
}

Answer 1

This can be done like this:

[
  {
    "operation": "shift",
    "spec": {
      "year": {
        "*": {
          "@(2,&)": "&"
        },
        "@": "&"
      }
    }
  },
  // just to sort
  {
    "operation": "sort",
    "spec": {
      "*": ""
    }
  }
]

"@(2,&)": "&" - traverses backwards up the source JSON and mapping the values ??from the top level to the value from "year".