Set on belongsTo

controllers.application.js

import Ember from 'ember';
import { get } from '@ember/object';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  init(){
    this._super(...arguments);
    this.model = get(this, 'store').createRecord('user', { name: 'Bob' });
    this.category = get(this, 'store').createRecord('category', { title: 'Blue' });
	},
   
   actions: {
     selectCategory(category){
       	console.log(category);
     		get(this, 'model').set('category', category);
       	console.log(get(this, 'model.category'));
       	console.log(get(this, 'model.category.title'));
     }
   }
    
});

models.category.js

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

export default Model.extend({
  title: attr('string'),
});

models.user.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: attr('string'),
  category: belongsTo('category'),
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

User : {{model.name}} <br>
Category : {{category.title}} <br>
<hr>
UserCategory : {{get model 'category.title'}}
<hr>
<button onclick={{action 'selectCategory' category}}>
	Select category
</button>

twiddle.json

{
  "version": "0.15.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.2.2",
    "ember-template-compiler": "3.2.2",
    "ember-testing": "3.2.2"
  },
  "addons": {
    "ember-data": "3.2.0"
  }
}