https://github.com/emberjs/data/issues/4067

adapters.survey-answer.js

import DS from "ember-data";

export default DS.JSONAPIAdapter.extend({
  urlForCreateRecord: function(type, snapshot) {
    var url = this._super(...arguments);
    
    var includes = snapshot.adapterOptions.includes;
    if (includes) {
      url += `?includes=${includes}`;
    }
    
    return url;
  }
});

application.route.js

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

server.map(function() {
  this.post('/survey-answers?', function(request) {
    var result = {
      data: {
        type: 'survey-answer',
        id: 1,
        relationships: {
          answer: {
            links: {
              related: "/survey-answers/1/answer",
              self: "/survey-answers/1/relationships/master-answer" 
            }
          }
        }
      }
    };
    
    if (request.queryParams.includes) {
      result.data.relationships.answer.data = {
        id: 1,
        type: "answer"
      }
    }
      
    return json(result);
  });
});

export default Ember.Route.extend({
  model: function() {
    let answer = this.store.push({
      data: {
        type: 'answer',
        id: 1,
        attributes: {
          text: "Le answer"
        }
      }
    });

    return this.store.createRecord('survey-answer', {
      answer
    });
  },
  
  actions: {
    save: function(surveyAnswer) {
      surveyAnswer.save({
        adapterOptions: {
          includes: "answer"
        }
      }).then((result) => {
        console.log(result.get("answer.text"));
      });
    }
  }
});

application.template.hbs

Answer: {{model.answer.text}}

<button {{action "save" model}}>save</button>

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.answer.js

import DS from 'ember-data';

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

models.survey-answer.js

import DS from 'ember-data';

export default DS.Model.extend({
  answer: DS.belongsTo()
});

twiddle.json

{
  "version": "0.4.17",
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.2.0",
    "ember-data": "2.2.1",
    "ember-template-compiler": "2.2.0",
    "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"
  }
}