Model Relationships are ObjectProxies not Models

components.demo-belongsto.js

import Ember from 'ember';
import Parent from '../models/parent';
const { service } = Ember.inject;

export default Ember.Component.extend({
  store: service(),
  parent: undefined,
  result: undefined,
  init() {
    this._super(...arguments);
    let store = this.get('store');
    let parent = store.createRecord('parent');
    let child = store.createRecord('child');
    parent.get('children').pushObject(child);
    this.set('parent', parent);
  },
  serializeModel(model) {
    // Is there a single way or serializing regardless of whether this is a parent or child?
    if (model.serialize) {
      return `Model: ${JSON.stringify(model.serialize())}`;
    } else if (model.content.serialize) {
      return `ObjectProxy: ${JSON.stringify(model.content.serialize())}`;
    }
  },
  actions: {
    serialize(model) {
      this.set('result', this.serializeModel(model));
    }
  }
});

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

models.child.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: 'child',
  parent: belongsTo('parent')
});

models.parent.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: 'parent',
  children: hasMany('child'),
});

routes.demo.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    let model = Parent.create();
    model.get('children').pushObjects(Child.create());
    return model;
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
{{outlet}}
{{demo-belongsto}}

templates.components.demo-belongsto.hbs

Parent: {{parent.name}}<button {{action 'serialize' parent}} style="color:green">serialize parent</button><br>

Parent's child: {{parent.children.firstObject.name}} (whose parent is {{parent.children.firstObject.parent.name}}) <button style="color:green"{{action 'serialize' parent.children.firstObject}}>serialize child</button><button style="color:red" {{action 'serialize' parent.children.firstObject.parent}}>serialize my parent</button><br>

Serialized output: {{result}}

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {}
}