https://github.com/emberjs/data/issues/3727#issuecomment-213663103

application.route.js

import Ember from 'ember';

export default Ember.Route.extend({
  beforeModel() {
    this.store.push({
      data: {
        type: 'folder',
        id: 2
      }
    });
  },

  model() {
    return this.store.push({
      data: {
        type: 'folder',
        id: 1,
        relationships: {
          parent: {
            data: {
              id: 2,
              type: 'folder'
            }
          }
        }
      }
    });
  }
});

application.template.hbs

parentId as set inside NodeMixin#ready() hook: {{model.parentId}}

mixins.node.js

import Ember from 'ember';

const { resolve } = Ember.RSVP;

export default Ember.Mixin.create({
  ready() {
    console.log(`ready ${this}`);
    
    var leParent = this.get("parent");

    resolve(leParent).then((parent) => {
      console.log(`parent: ${parent}`);

      if (parent) {
        var parentId = parent.get("id");
        this.set('parentId', parentId);
      }
    }); 
  }
});

models.folder.js

import DS from 'ember-data';
import NodeMixin from '../mixins/node';

export default DS.Model.extend(NodeMixin, {
  name: DS.attr(),
  parent: DS.belongsTo('folder', { inverse: null, async: true })
});

twiddle.json

{
  "version": "0.5.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.1.1",
    "ember-data": "2.5.2",
    "ember-template-compiler": "2.1.1"
  }
}