New Twiddle

adapters.post.js

import JSONAPIAdapter from 'ember-data/adapters/json-api';
export default class PostAdapter extends JSONAPIAdapter {
  async findRecord(store, type, id) {
    switch (id) {
      case '1':
        return {
          "data": {
            "id": "1",
            "type": "posts",
            "links": {
              "self": "http:\/\/localhost:3000\/posts\/1"
            },
            "attributes": {
              "title": "Foo"
            },
            "relationships": {
              "author": {
                "links": {
                  "self": "http:\/\/localhost:3000\/posts\/1\/relationships\/author",
                  "related": "http:\/\/localhost:3000\/posts\/1\/author"
                },
                "data": null
              }
            }
          }
        };

      case '2':
        return {
          "data": {
            "id": "2",
            "type": "posts",
            "links": {
              "self": "http:\/\/localhost:3000\/posts\/2"
            },
            "attributes": {
              "title": "Bar"
            },
            "relationships": {
              "author": {
                "links": {
                  "self": "http:\/\/localhost:3000\/posts\/2\/relationships\/author",
                  "related": "http:\/\/localhost:3000\/posts\/2\/author"
                }
              }
            }
          }
        };
    }
  }
  
  async findBelongsTo(store, snapshot, url) {
		return {
      data: null,
      meta: {},
      links: {},
    };
  }
}

controllers.application.js

import Ember from 'ember';
import { inject as service } from '@ember/service';

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

helpers.v\.js

import { helper } from '@ember/component/helper';

export default helper(function v(params/*, hash*/) {
  switch (params[0]) {
    case undefined:
      return '[[UNDEFINED]]';
    case null:
      return '[[NULL]]';
    default:
      return params[0];
  }
});

models.author.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'),
  posts: hasMany('post', { inverse: 'author' }),
});

models.post.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import { belongsTo, hasMany } from 'ember-data/relationships';

export default Model.extend({
  title: attr('string'),
  author: belongsTo('author', { inverse: 'posts' }),
});

routes.application.js

import Ember from 'ember';
import { returnSnapshot } from '../serializers/application';
import { all } from 'rsvp';

export default Ember.Route.extend({
  async model() {
    const post1 = await this.store.findRecord('post', 1, { include: 'author' });
    const post2 = await this.store.findRecord('post', 2);
    
    await post2.belongsTo('author').load();
    
    const snapshot1 = post1.serialize({ [returnSnapshot]: true });
    const snapshot2 = post2.serialize({ [returnSnapshot]: true });
    console.log(snapshot2);
    
    console.log(snapshot1.belongsTo('author')); // logs `null`
    console.log(snapshot2.belongsTo('author')); // logs `undefined`, expected `null`.
    
    return {
      posts: [post1, post2],
      authors: [
        snapshot1.belongsTo('author'),
        snapshot2.belongsTo('author')
      ]
    };
  }
});

serializers.application.js

import JSONAPISerializer from 'ember-data/serializers/json-api';
export const returnSnapshot = Symbol();

export default JSONAPISerializer.extend({
  serialize(snapshot, options) {
    if (options[returnSnapshot]) {
      return snapshot;
    }

    return this._super(...arguments);
  },
});

templates.application.hbs

<h3>posts</h3>

<ul>
  {{#each this.model.posts as |post|}}
    <li>{{post.id}} {{post.title}}</li>
  {{/each}}
</ul>

<h3>authors</h3>

<ul>
  {{#each this.model.authors as |author|}}
    <li>{{v author}}</li>
  {{/each}}
</ul>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.16.0"
  }
}