ember JSONAPISerializer methods not working

I have an underscored API response and in my JSONSerializer i’m overriding keyForAttribute but it is not working and i placed a debugger in keyForAttribute function it is not firing at all.

I also tried adding attr properties but it is not working as well.

here is my Serializer:

import DS from "ember-data";
import { underscore } from "@ember/string";

export default DS.JSONSerializer.extend({
  keyForAttribute(attr) {
    return underscore(attr);
  },


 attrs: {
   documentType: 'type',
   coverUrl: "cover_url",
   mainTopic: "main_topic",
   publicationDate: "publication_date",
   globalReads: "global_reads",
   localReads: "local_reads"
 },
  normalizeResponse(store, primaryModelClass, payload, id, requestType) {
    return {
      data: payload.documents.map((item, index) => {
        return {
          id: index,
          type: "document",
          attributes: item
        };
      })
    };
  }
});

and here is my model

import DS from 'ember-data';

export default DS.Model.extend({
  title: DS.attr('string'),
  coverUrl: DS.attr('string'),
  authors: DS.attr(),
  institution: DS.attr('string'),
  documentType: DS.attr('string'),
  publicationDate : DS.attr('date'),
  mainTopic: DS.attr('string'),
  globalReads: DS.attr('number'),
  localReads: DS.attr('number'),
});

and here is part of my API response:

    {"title": "World War 2",

    "frequency": "Monthly",
    "type": "THESIS",
    "languages": [
        "ENGLISH",
        "ARABIC"
    ],
    "doi": "10.1000/182",
    "descriptors": [
        "ARTS",
        "PSYCHOLOGY"
    ],
    "keywords": [
        "history",
        "adolf hitler",
        "war"
    ],
    "affiliation": "history",
    "isbn": "978-92-95055-02-5",
    "institution": "History Palace",
    "eissn": 2312,
    "pissn": 1595,
    "document_id": "1",
    "publication_date": 1512346539773,
    "main_topic": "ARTS",
    "document_abstract": "some document abstract here",
    "cover_url": "http://ift.tt/2BQ9khs",
    "content_url": "http://ift.tt/2BQ9khs",
    "global_reads": 244,
    "local_reads": 102,
    "translated_title": "الحرب العالمية الثانية",
    "dewey_number": "900-999",
    "program_type": "MASTER",
    "publishing_country": "NEW_ZEALAND"
},

any help would be very appreciated.

Thank you.