https://github.com/emberjs/data/issues/3784

application.route.js

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

server.map(function() {
  this.get('/file-types/1', function() {
    return json({
      'file-types': [{
        id: 1,
        name: "GIF"
      }]        
    });
  });
  this.get('/returns-file-type-from-other-api/1', function() {
    return json({
      fileTypes: [{
        id: 1,
        name: "GIF"
      }]        
    });
  });
  this.get('/files/1', function() {
    return json({
      files: [{
        id: 1,
        name: 'ape.gif',
        type: 1
      }]        
    });
  });
});

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

application.template.hbs

<div style="font-size: 18px;font-weight:bold">Bug in JSONAPISerializer in combination with RESTSerializer</div>

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

Check console for handled requests!

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}`);
  }

  server.map(function() {
    this.get('/people', returnJSON({
      data: []
    }));
  });
};

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

models.file-type.js

import DS from 'ember-data';

export default DS.Model.extend({
  name: DS.attr()
});

models.file.js

import DS from 'ember-data';

const { attr, belongsTo } = DS;

export default DS.Model.extend({
  name: attr(),
  type: belongsTo('file-type', { async: true })
});

serializers.file-type.js

export default DS.RESTSerializer.extend({
  isNewSerializerAPI: true
});

serializers.file.js

export default DS.RESTSerializer.extend({
  isNewSerializerAPI: true,
  
  normalize(typeClass, hash, prop) {
    if(prop === 'files') {
      hash.links = {
        type: `/returns-file-type-from-other-api/${hash.type}`
      }
    }
    
    //calls RESTSerializer normalize();
    //RESTSerializer normalizer will call JSONAPISerializer normalize() + return that as result
    const jsonApiResponse = this._super(...arguments);
    
    /* 
    	JsonApiResponse has one relationship: type. Type object has following structure:
      
      type: {
        data: {
          id: "1",
          type: "file-type"
        },
        links: {
          related: "/returns-file-type-from-other-system/1"
        }
      }
    */
     
    //Because both `data` and `links` object exist for the `type` relationship, /file-types/ will be called. Only `link` object was expected.
    //So we remove the data object!
    delete jsonApiResponse.data.relationships.type.data; //this delete statement should not be needed!
    
    return jsonApiResponse;
  }
});

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {
      "ds-references": true
    }
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.4.3",
    "ember-data": "2.4.3",
    "ember-template-compiler": "2.4.3",
    "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"
  }
}