Ember Data promise proxy

adapters.bar\.js

import RESTAdapter from 'ember-data/adapters/rest';

export default class FooAdapter extends RESTAdapter {
  ajax(path, method, options) {    
    const id = path.match(/\d+/)[0];
    return {
      bar: {
        id: id,
        name: `Bar${id}`
      }
    }
  }	 
}

adapters.foo\.js

import RESTAdapter from 'ember-data/adapters/rest';

export default class FooAdapter extends RESTAdapter {
  ajax(path, method, options) {    
    return {
      foo: {
        id: 1,
        name: 'Foo 1',
        bar: 1
      }
    }
  }	 
}

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';

export default class ApplicationController extends Controller {
  @action
  save() {
    // A fully resolved model
    console.log('foo', this.foo);
    
    // A *fulfilled* promise proxy object for a model
    console.log('foo.bar', this.bar);

    // Clear out the relationship
    this.foo.set('bar', null);
    
    // Set the relationship back using the
    // *fullfilled promise proxy object*
    this.foo.set('bar', this.bar);
    
    // Save...
    this.foo.save();
  }
}

models.bar\.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';

export default class extends Model {
  @attr() name;
}

models.foo\.js

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

export default class extends Model {
  @attr() name;
  @belongsTo('bar') bar;
}

routes.application\.js

import Route from '@ember/routing/route';
import { hash } from 'rsvp';

export default class ApplicationRoute extends Route {
  model() {
    return this.store.find('foo', 1);
  }
  
  setupController(controller, model) {
    controller.foo = model;
    
    // Resolve bar ahead of time for this example.
    
    controller.bar = controller.foo.bar;
    
    // This fixes the issue, but why?
    // controller.bar = this.store.findRecord('bar', 1);
  }
}

serializers.bar\.js

import RESTSerializer from 'ember-data/serializers/rest';

export default class BarSerializer extends RESTSerializer {}

serializers.foo\.js

import RESTSerializer from 'ember-data/serializers/rest';

export default class FooSerializer extends RESTSerializer {
  serialize() {
    const json = super.serialize(...arguments);
    console.log('serializing foo', json);
  }
}

templates.application\.hbs

Click save and check the console.<br>
Try this with ED 3.10 and then ED 3.18<br><br>

Then also look at the conment in routes/application.js
<br><br>

<button type="button" {{on "click" this.save}}>
  Save
</button>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-data": "3.10.0"
  }
}