Embedded Records

adapters.foo\.js

import RESTAdapter from 'ember-data/adapters/rest';
const { stringify } = JSON;

export default class FooAdapter extends RESTAdapter {
  ajax(path, method, options) {    
    if (method === 'PUT' || method === 'POST') {
     	console.log(method, stringify(options.data, null, 2)) 
    }
    
    return {
      foo: {
        id: 1,
        name: 'Foo 1',
        bars: [{
          id: 1,
          name: 'Bar 1'
        }]
      }
    }
  }	
}

controllers.application\.js

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

export default class ApplicationController extends Controller {
  @action
  save() {
    this.foo.save(); 
  }
}

models.bar\.js

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

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

models.foo\.js

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

export default class FooModel extends Model {
  @attr() name;
  @hasMany('bar') bars;
}

routes.application\.js

import Route from '@ember/routing/route';

export default class ApplicationRoute extends Route {
  model() {
    return this.store.findRecord('foo', 1); 
  }
  
  setupController(controller, model) {
    controller.foo = model;
  }
}

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';
import EmbeddedRecordsMixin from 'ember-data/serializers/embedded-records-mixin';  

export default class FooSerializer extends RESTSerializer.extend(EmbeddedRecordsMixin) {
  attrs = {
    bars: {
      embedded: 'always'
    }
  };

	serializeHasMany() {
    console.log('here');
    super.serializeHasMany(...arguments);
  }
}

templates.application\.hbs

<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": "3.18.0"
  }
}