error while loading route: - why am I not retrieving data form rails?

I'm trying to set up the basics of an application using Ember on Rails, and I'm getting this error:

Error while processing route: posts

It would be helpful if Ember said something about the error, but it hasn't. Here is the code in question (I believe I am including all parts needed).

// PostsRoute

App.PostsRoute = Ember.Route.extend({
    model: function(){
        return this.store.find('post');
        // return [{title: 'fixture'}, {title: '2'}]
    }

})

// Router.

App.Router.map(function() {
  // location: 'auto',
  // rootURL: '/',
  this.resource('posts', { path: '/' })

});

// Routes.rb
  resources :posts do 
    resources :comments
  end

// Post model - Ember
App.Post = DS.Model.extend({
    title: DS.attr('string'),
    description: DS.attr('string'),

    imageFileName: DS.attr('string'),
    imageContentType: DS.attr('string'),
    imageFileSize: DS.attr('number'),
    imageUpdatedAt: DS.attr('date'),

    createdAt: DS.attr('date'),
    updatedAt: DS.attr('date'),

    userId: DS.attr('number'),
});


// Posts controller.rb

class PostsController < ApplicationController
  before_action :set_post, only: [:show, :edit, :update, :destroy]

  respond_to :html

  def index
    @posts = Post.all
    puts "in PostsController index. "
    puts @posts
    respond_with(@posts)
  end

  ...

// Post Serializer
class PostSerializer < ActiveModel::Serializer
  attributes :id, :title, :description, :created_at, :updated_at, :user_id, :image_file_name, :image_content_type, :image_file_size, :image_updated_at
end

I'm trying to understand what needs to be set up for Ember to receive data from the Rails API. Rails-wise, it's returning data fine.

From what I understand, here is how the app goes. Ember checks the route (here, PostsRoute) first. The route is expected to have a 'model', so it looks at the Model property. Here, the model property makes a call to the Store to get all posts. i.e. this.store.find('post'). [replacing this call with a static array seems to work fine.]

This makes a call to the Rails PostsController's index function. I can confirm that this function is reached because of the print statements I put there. The index function returns a variable with the activerecord post models.

From here, the posts need to be JSON-ified for Ember to be able to use it. So they are put through a serializer, which, here, is post_serializer.rb. This decides what properties to send as JSON form.

I suspect that there is some issue with the way I am handling this serialization part, since I have confirmed that the posts have actually been fetched in the index function of PostsController.rb, but have failed (I believe) to make the journey back to Ember.

What I don't understand is why. What is it that I'm missing?

If it is any issue, there are some has_many relationships I have not included yet because I first wanted to get this going. And for completeness, here is the full error as per chrome's console:

Error while processing route: posts ember.js?body=1:15376
logToConsole ember.js?body=1:15376
logError ember.js?body=1:26315
defaultActionHandlers.error ember.js?body=1:26272
triggerEvent ember.js?body=1:26363
trigger ember.js?body=1:46876
Transition.trigger ember.js?body=1:46721
(anonymous function) ember.js?body=1:46526
tryCatch ember.js?body=1:47310
invokeCallback ember.js?body=1:47322
publish ember.js?body=1:47293
publishRejection ember.js?body=1:47235
(anonymous function) ember.js?body=1:29438
DeferredActionQueues.invoke ember.js?body=1:682
DeferredActionQueues.flush ember.js?body=1:752
Backburner.end ember.js?body=1:138
Backburner.run ember.js?body=1:193
run ember.js?body=1:18226
hash.error ember-data.js?body=1:1886
fire jquery.js?body=1:3120
self.fireWith jquery.js?body=1:3232
done jquery.js?body=1:9278
callback jquery.js?body=1:9686