JSLT

I’ve a GraphQL service and a Spring Boot application. The latter consumes the former. In the application, I transform the retrieved JSON with the help of JSLT. Today, I wrote a JSLT program to do de-duplication and array intersection. Below is the context:

My GraphQL query:

query MyQuery(...) {
  arrayA: someModel(...) {
    someInnerModel {
      id
    }
  },
  arrayB: someModel(...) {
    someInnerModel {
      id
    }
  }
}

Expected JSON response from GraphQL service:

{
  "data": {
    "arrayA": [
      {
        "someInnerModel": {
          "id": 1
        }
      },
      {
        "someInnerModel": {
          "id": 2
        }
      }
    ],
    "arrayB": [
      {
        "someInnerModel": {
          "id": 1
        }
      },
      {
        "someInnerModel": {
          "id": 3
        }
      }
    ]
  }
}

We want the final JSON to look like:

[
  {
    "id": 1
  }
]

JSLT has built-in functions but they are very simple ones. At first glance, it may seem impossible to do de-duplication and array intersection. However, it is possible to do these tasks with good knowledge of JavaScript objects. Without further ado, here’s my JSLT snippet:

def removeDuplicates(array)
  let object = {for ($array) string(.) : .}
  [for ($object) .value]

def getIntersection(array1, array2)
  let ids1 = (removeDuplicates([for ($array1) .someInnerModel.id]))
  let ids2 = (removeDuplicates([for ($array2) .someInnerModel.id]))
  [for ($ids1) . if (contains(., $ids2))]

let ids = (getIntersection(.data.arrayA, .data.arrayB))
[for ($ids)
{
  "id": .
}]

Some notes:

{for ($array) string(.) : .} transforms [1, 2, 2, 3] into {"1": 1, "2": 2, "3": 3}.
[for ($object) .value] transforms {"1": 1, "2": 2, "3": 3} into [1, 2, 3].
[for ($array1) .someInnerModel.id] transforms [{"someInnerModel":{"id":1}}] into [1].
contains(., $ids2) returns true if the value represented by . exists in $ids2.
[for ($ids1) . if (contains(., $ids2))] is like Python’s list comprehension i.e. [x for x in range(5) if (x in ids2)].
getIntersection does not return elements from $array1. It returns elements from $ids1.
I surrounded my variable definitions with parentheses because of this bug.