0_adapter.js

// adapters/comment.js

import ApplicationAdapter from './application';
import Ember from 'ember';
import UrlTemplates from "ember-data-url-templates";

export default ApplicationAdapter.extend(UrlTemplates, {
  urlTemplate: "{+host}/posts/{postId}/comments{/id}",
  adapterContext: Ember.inject.service(),

  urlSegments: {
    postId() {
      return this.get('adapterContext.post.id');
    },
  }
});

1_router.js

import Ember from 'ember';
import config from './config/environment';

var Router = Ember.Router.extend({
  location: config.locationType
});

Router.map(function() {
  this.resource('post', { path: 'posts/:post_id' }, function() {
    this.resource('post.comments', {path: 'comments'}, function() {
      // your routes here
    });
  });
});

export default Router;

2_route.js

// routes/post/comments/index.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    let post = this.modelFor('post');
    this.get('adapterContext').setContext({post});

    return this.store.findAll("comment");
  }
});