Loading associated resources in Ember - ruby-on-rails

Question

I'm working on my second Ember project, and first using Rails on the backend. I'm struggling with loading associated data through my API in a nested route. The association is simple: a folder has many media_files in my backend (I'm aware snake case is against convention but trying to work around it).
When I do the following, I get the correctly nested route (folders/show/media_files) and no complaints when loading the template, but the data is empty in the console and doesn't render in the template.
Thank you for your time.
Here is my routing:
Router.map(function() {
this.route('folders', function(){
this.route('show', {
path: ':folder_id'
}, function() {
this.route('media_files', {resetNamespace: true}, function (){
});
});
});
});
Here are my associations:
Folder:
import DS from 'ember-data';
export default DS.Model.extend({
name: DS.attr('string'),
media_files: DS.hasMany('media_file')
});
Media Files:
import DS from 'ember-data';
export default DS.Model.extend({
name: DS.attr('string'),
duration: DS.attr('number'),
createdAt: DS.attr('date'),
folder: DS.belongsTo('folder')
});
Here is the call to the media files index:
import Ember from 'ember';
export default Ember.Route.extend({
model: function(){
return this.modelFor("folders/show").get("media_files");
}
});
My URL is folders/folder_id/media_files, same as in my API. The JSON there looks like this:
{
media_files: [
{
id: 513009,
url: null,
project_id: 999,
batch_id: 1268,
duration: 30556,
rush: false.....