adapter.js

import UrlTemplates from "ember-data-url-templates";

export default DS.RESTAdapter.extend(UrlTemplates, {
  urlTemplate: '{+host}/posts/{postId}/comments{/id}',
  findUrlTemplate: '{+host}/comments/{id}',
  createRecordUrlTemplate: '{+host}/users/{userId}/comments',

  currentUser: Ember.inject.service(),

  urlSegments: {
    postId(type, id, snapshot, query) {
      return snapshot.belongsTo('post', { id: true });
    },

    userId() {
      return this.get('currentUser.id');
    }
  }
});

I fixed a small typo in my fork, if you're interested: https://gist.github.com/GabrielCW/da2c95ace69913885513