. . .
    public function getResourceKey(): string {
        return 'neliti-volumes';
    }
. . .

. . .
    public function getResourceKey(): string {
        return 'neliti-publications';
    }
. . .

{
  "data": [
    {
      "type": "neliti-volumes",
      "id": "088f4670-8993-4ffc-86e4-58036442cebe",
      "attributes": {
        "volume": "BB",
        "number": "3",
      },
      "relationships": {
        "neliti-publication": { // this one is from the url that mapped with the transformers
          "data": {
            "type": "nelitiPublication", // this one should be neliti-publications that get from getResourceKey()
            "id": "39676c75-6e03-11ea-9660-ed7fb61131c5"
          }
        }
      }
    },
    . . .
  ]
}