https://github.com/emberjs/data/issues/4359

adapters.application.js

import DS from "ember-data";

export default DS.JSONAPIAdapter.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) {
    let msg = `undhandled request ${verb} ${path}`;
    console.log(msg);
    return [404, {}, msg];
  }
};

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

models.person.js

import DS from "ember-data";

const { Model, attr, hasMany } = DS;

export default Model.extend({
  name: attr(),
  friends: hasMany("person")
});

routes.application.js

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

server.map(function() {
  this.get('/people/1', function() {
    return json({
      data: {
        type: 'person',
        id: 1,
        attributes: {
          name: 'George Michael Bluth'
        },
        relationships: {
          friends: {
           data: [{
             id: 2,
             type: 'person'
           }]
          }
        }
      }
    });
  });
  
  this.get('/people/2', function () {
    return json({
      data: {
        type: 'person',
        id: 2,
        attributes: {
          name: 'Maebe Bluth'
        },
        relationships: {
          friends: {
           data: [{
             id: 1,
             type: 'person'
           }]
          }
        }
      }
    });
  });
});

export default Ember.Route.extend({
  model: function() {
    return this.store.findRecord('person', 1);
  },
  
  afterModel(person) {
    
    let reference = person.hasMany('friends');
    console.log('value before load', reference.value());
    reference.load().then(function() {
      console.log('value after load', reference.value());
    }).catch(err => {
      console.log('err', err);
    });
  }
});

serializers.application.js

import DS from "ember-data";

export default DS.JSONAPISerializer.extend();

templates.application.hbs

Name: {{model.name}}

twiddle.json

{
  "version": "0.7.2",
  "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": "canary",
    "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"
  }
}