Why are the banks not set as relationships?

application.adapters.application.js

import DS from 'ember-data';
//import ENV from '../config/environment';

export default DS.JSONAPIAdapter.extend({
    host: '',// is configured in environment
    namespace: ''// is configured in environment,
});

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Emberry Twiddle'
});

application.controllers.index.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'foo'
});

application.models.user.js

import Ember from 'ember';
import DS from 'ember-data';

export default DS.Model.extend({
  email: DS.attr('string'),
  realName: DS.attr('string'),
  bankId: DS.belongsTo('bank', {async: true})
});

application.router.js

import Ember from 'ember';
import config from './config/environment';

const Router = Ember.Router.extend({
  location: config.locationType
});

Router.map(function() {
  this.route('index', {path: '/'});
});

export default Router;

application.routes.index.js

import Ember from 'ember';

export default Ember.Route.extend({
  model: function () {
    return this.store.findAll('user');
  }
});

application.serializers.user.js

import DS from 'ember-data';

export default DS.JSONAPISerializer.extend({
  keyForRelationship (key, relationship, method) {
    console.info('JSONAPI serializing user:',key, relationship, method);
    /**
     * Map bankId ('bank-id') to bank.
     */
    if (key === 'bankId') {
      if (method === 'serialize') {
        return 'bank';
      } else {
        return 'bank';
      }
    }
  },

  // Just to map bank-id back to user-record on userRecord.save()
  serializeBelongsTo (record, json, relationship) {
    let bankId = record.get('bankId.id');
    if (bankId && json.attributes) {
      json.attributes['bank-id'] = json.attributes['bank-id'] || bankId;
    }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
{{#each model as |user|}}
<p>{{user.id}}</p>
{{/each}}

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.10/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/1.13.15/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.0/ember-template-compiler.js"
  }
}