Fail to fetch

adapters.application.js

import Adapter from "ember-data/adapters/json-api";

export default Adapter.extend();

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  personWithoutGroup: Ember.computed(function() {
    return this.get('store').findRecord('person', 1);
  }),
  personWithGroup: Ember.computed(function() {
    return this.get('store').findRecord('person', 2);
  }),
  
  actions: {
    setgroup(record) {
      this.get('store').findRecord('group', 1).then(g => record.set('group', g));
    },
    
    unsetgroup(record) {
      record.set('group', null);
    },
    
    reload(record) {
      this.get('store').peekRecord('person', record.get('id')).reload();
    },
    
    query(record) {
      this.get('store').findRecord('person', record.get('id'));
    }
  }
});

initializers.pretender.js

export function returnJSON(status, body) {
  return json(...arguments);
};

export function json(status, body) {
  if (arguments.length === 1) {
    body = status;
    status = 200;
  }
  
  return [
    status,
    { "Content-Type": "application/json" },
    JSON.stringify(body)
  ];
};

export const server = new Pretender();

export function initialize() {
  server.handledRequest = function(verb, path, request) {
    console.log(`handled request to ${verb} ${path}`);
  }

  server.unhandledRequest = function(verb, path, request) {
    console.log(`undhandled request ${verb} ${path}`);
  }
};

export default {
  name: 'pretender',
  initialize: initialize
};

models.group.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.person.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'),
  group: belongsTo('group', { inverse: null }),
});

routes.application.js

import Ember from 'ember';
import { server, json } from '../initializers/pretender';

server.map(function() {
  this.get('/groups/1', function() {
    return json({
      data: {
        type: 'group',
        id: 1,
        attributes: {
          name: 'This is a group'
        }
      }
    });
  });
  
  this.get('/groups/2', function() {
    return json({
      data: {
        type: 'group',
        id: 2,
        attributes: {
          name: 'This is a group 2'
        }
      }
    });
  });
  
  this.get('/people/1', function() {
    return json({
      data: {
        type: 'person',
        id: 1,
        attributes: {
          name: 'Person without group'
        }
      }
    });
  });
  
  this.get('/people/2', function() {
    return json({
      data: {
        type: 'person',
        id: 2,
        attributes: {
          name: 'Person with group'
        },
        relationships: {
          group: {
            data: {
              type: 'group',
              id: 1,
            }
          }
        }
      }
    });
  });
});

export default Ember.Route.extend({
});

serializers.application.js

import Serializer from "ember-data/serializers/json-api";

export default Serializer.extend();

templates.application.hbs

Name: {{personWithoutGroup.name}}<br>
Group: {{if personWithoutGroup.group personWithoutGroup.group.name 'No group'}}<br>
<button {{action 'setgroup' personWithoutGroup}}>Set</button>
<button {{action 'reload' personWithoutGroup}}>Reload</button>
<button {{action 'query' personWithoutGroup}}>Query</button>
<br>
<br>
Name: {{personWithGroup.name}}<br>
Group: {{if personWithGroup.group personWithGroup.group.name 'No group'}}<br>
<button {{action 'unsetgroup' personWithGroup}}>Unset</button>
<button {{action 'reload' personWithGroup}}>Reload</button>
<button {{action 'query' personWithGroup}}>Query</button>

twiddle.json

{
  "version": "0.10.4",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.16.0",
    "ember-template-compiler": "release",
    "route-recognizer": "https://rawgit.com/tildeio/route-recognizer/56f5fcec6ae58d8e86b5dc77609809fb91198142/dist/route-recognizer.js",
    "FakeXMLHttpRequest": "https://rawgit.com/pretenderjs/FakeXMLHttpRequest/23c3a96b5b24f1bfe595867437e4f128a29c2840/fake_xml_http_request.js",
    "pretender": "https://rawgit.com/pretenderjs/pretender/c6de9afe18b1472aded2592f5a80ad9a26a0e262/pretender.js"
  },
  "addons": {
    "ember-data": "2.16.3"
  }
}