New Twiddle

adapters.genre.js

import DS from 'ember-data';

export default DS.Adapter.extend({
  findAll() {
    return {
      genres: [
        {"id":1,"name":"Action"},
        {"id":2,"name":"Adventure"},
        {"id":3,"name":"Animation"},
        {"id":4,"name":"Comedy"},
      ]
    };
  }
});

adapters.movie.js

import DS from 'ember-data';

export default DS.Adapter.extend({
  findAll() {
    return {
      results: [{
        "id":1,
        "title": "Venom",
        "genre_ids": [1,2,3],
      },{
        "id":2,
        "title": "Example Movie",
        "genre_ids": [3,4],
      }],
    };
  }
});

models.genre.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  name: attr('string'),
});

models.movie.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  title: attr('string'),
  genres: hasMany('genre'),
});

routes.application.js

import Ember from 'ember';
import RSVP from 'rsvp';

export default Ember.Route.extend({
  model() {
    return RSVP.hash({
      genres: this.store.findAll('genre'),
      movies: this.store.findAll('movie'),
    }).then(x => x.movies);
  }
});

serializers.genre.js

import DS from 'ember-data';

export default DS.JSONSerializer.extend({
  normalizeArrayResponse (store, primaryModelClass, payload, id, requestType) {
    return this._super(store, primaryModelClass, payload.genres, id, requestType);
  }
});

serializers.movie.js

import DS from 'ember-data';
import {singularize} from 'ember-inflector';

export default DS.JSONSerializer.extend({
  normalizeArrayResponse (store, primaryModelClass, payload, id, requestType) {
    return this._super(store, primaryModelClass, payload.results, id, requestType);
  },
  keyForRelationship(key, typeClass, method) {
    return `${singularize(key)}_ids`;
  }
});

templates.application.hbs

<h1>{{model.length}} movies</h1>

{{#each model as |movie|}}
  <h3>{{movie.id}} - {{movie.title}}</h3>
  Has {{movie.genres.length}} genres:
  {{#each movie.genres as |genre|}}
  	{{genre.name}},
  {{/each}}
{{/each}}

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}