Computed BelongsTo Ids

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  store: Ember.inject.service(),
  
  init() {
    this._super(...arguments);
    const crust1 = this.get('store').createRecord('pizza-crust', { 
      id: 'crust-1' 
    });
    const crust2 = this.get('store').createRecord('pizza-crust', { 
      id: 'crust-2' 
    });
    const pizza = this.get('store').createRecord('pizza', { 
      id: 'pizza-1', 
      crust: crust1 
    });
    this.setProperties({ pizza, crust1, crust2 });
  },
  
  actions: {
    toggleCrust() {
      if (this.get('pizza.crustId') === this.get('crust1.id')) {
        this.set('pizza.crust', this.get('crust2'));
      } else {
        this.set('pizza.crust', this.get('crust1'));
      }
    },
    
    removeCrust() {
      this.set('pizza.crust', null);
    }
  }
});

mixins.with-belongs-to-ids.js

import Ember from 'ember';

const { computed } = Ember;

export default Ember.Mixin.create({
  init() {
  	this._super(...arguments);

  	this.eachRelationship((name, { kind }) => {
    	if (kind === 'belongsTo') {
      	const computedId = computed(name, function() { 
          return this.belongsTo(name).id(); 
        });
        Ember.defineProperty(this, `${name}Id`, computedId);
    	}
  	});
	}
});

models.pizza-crust.js

import DS from 'ember-data';

import WithBelongsToIds from 'app/mixins/with-belongs-to-ids';

export default DS.Model.extend(WithBelongsToIds, {
});

models.pizza.js

import DS from 'ember-data';

import WithBelongsToIds from 'app/mixins/with-belongs-to-ids';

export default DS.Model.extend(WithBelongsToIds, {
  crust: DS.belongsTo('pizza-crust'),
  toppings: DS.hasMany('pizza-topping')
});

templates.application.hbs

<button {{action "toggleCrust"}}>Toggle Crust</button>
<button {{action "removeCrust"}}>Remove Crust</button>
<p>pizza.id: {{pizza.id}}</p>
<p>pizza.crustId: {{pizza.crustId}}</p>
<p>pizza.crust.Id: {{pizza.crust.id}}</p>

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": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}