Why am I getting the error Uncaught TypeError: Object post has no method '_create' when trying to create a new post?

posts.js.coffee

Blember.Post = DS.Model.extend
  title: DS.attr('string'),
  isCompleted: DS.attr('boolean')

Blember.Post.FIXTURES = [
  {
    id: 1,
    title: 'Hello World',
    body: 'This is my first post',
    isCompleted: true
  },
  {
    id: 2,
    title: 'Hello Ember',
    body: 'This is my first post',
    isCompleted: false
  }
]

posts_controller.js.coffee

Blember.PostsController = Ember.ArrayController.extend
  actions:
    createPost: ->
      title = @get('newTitle')
      return if !title.trim()

      post = @store.createRecord 'post', {
        title: title, isCompleted: false
      }

      @set('newTitle', '')
      post.save()

posts_controller.rb

class PostsController < ApplicationController
  POSTS = [
    {title: "Hello World", body: "This is a body", is_completed: true},
    {title: "Another World", body: "Doodle, strudel", is_completed: false}
  ]

  def index
    respond_to do |format|
      format.html
      format.json {
        render json: POSTS
      }
    end
  end

  def create
    POSTS << params[:post]
  end

end