New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import {computed, get, action, set} from "@ember/object";

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';


}

controllers.planbook\.js

import Controller from '@ember/controller';
import {computed, get, action, set} from "@ember/object";

export default class PlanbookController extends Controller {
  
  @computed("model.planbook.relationships.courses.data")
  get courseRelationships() {
    return get(this, "model.planbook.relationships.courses.data") 
  }
  

  @action
	addCourseRelationship(){
    console.log('Add action')
   	set(this.model.planbook.relationships.courses, "data", [
      {id: 1, type: "course"}, 
      {id: 2, type: "course"}
    ])
  }

}

router\.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route("planbook", {path: "/planbook"})
});

export default Router;

routes.application\.js

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

export default Route.extend({
  model(){
    return new Promise((resolve, reject) => {
    resolve({
    	planbook: {
        relationships: {
          courses: {
            data: [{id: 1, type: "course"}]
          }
        }
      }
    })      
    })

  }
});

routes.planbook\.js

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

export default Route.extend({
});

templates.application\.hbs

{{outlet}}

templates.planbook\.hbs

{{log "courseRelationships" @model.planbook.relationships.courses.data}}
{{log "courseRelationships CP" this.courseRelationships}}

<button {{on "click" this.addCourseRelationship}}>Add Course</button>

twiddle\.json

{
  "version": "0.17.0",
  "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/3.3.1/jquery.js",
    "ember": "3.17.0",
    "ember-template-compiler": "3.17.0",
    "ember-testing": "3.17.0"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}