https://github.com/emberjs/data/issues/4666

adapters.application.js

import Adapter from "ember-data/adapters/rest";

export default Adapter.extend();

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
};

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(),
  type: attr(),
  sibling: belongsTo('person', { polymorphic: true })
});

routes.application.js

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

server.map(function() {
  this.get('/people/1', function() {
    return json({
      person: {
        id: 1,
        name: 'George Michael',
        type: 'mr. manager',
        sibling: 2,
        sibling_type: 'person'
      },
      people: [{
        id: 2,
        name: 'Maeby',
        type: 'troublemaker'
      }]
    });
  });
});

export default Ember.Route.extend({
  model: function() {
    return this.store.findRecord('person', 1);
  }
});

serializers.application.js

import Serializer from "ember-data/serializers/rest";

export default Serializer.extend();

templates.application.hbs

Name: {{model.name}} <br>
Type: {{model.type}} <br>
Sibling: {{model.sibling.name}} <br>
Sibling Type: {{model.sibling.type}} <br>

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": "release",
    "ember-data": "release",
    "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": {}
}