How can I request the relationships of a resource without using a filter?

I have a backend that follows the JSON API specification.

In my Ember app, I do something like this:

model() {
  return this.store.query('category', { filter: { forum: 'main' } });
}

This works well and the request sent to the server is GET /categories?filter[forum]=main. My app gets all the categories from the forum with ID main.

Now, instead of the previous request, I would like to make a GET /forums/main/categories from the model. How can this be done in Ember with Ember Data?

Here's something I tried with Ember AJAX:

ajax: Ember.inject.service(),

model() {
  return Ember.RSVP.hash({
    categories: this.get('ajax').request('/forums/main/categories'),
  });
}

The request works and the correct data is returned from the server. But Ember Data just doesn't know about it and I can't use the model in my template. How can I make Ember AJAX work with Ember Data?

The Ember AJAX GitHub page proposes to write something like that:

import DS from 'ember-data';
import AjaxServiceSupport from 'ember-ajax/mixins/ajax-support';

export default DS.JSONAPIAdapter.extend(AjaxServiceSupport);

http://ift.tt/2dZFLj1

But it doesn't seem to change anything.

Any help would be appreciated. Thank you.