App.ArticleAdapter =DS.FixtureAdapter.extend({}); extend undefined in ember js-Uncaught TypeError: Cannot read property 'extend' of undefined

i am new to emberjs. when i am trying to extend DS.FixtureAdapter i am getting error like'Uncaught TypeError: Cannot read property 'extend' of undefined' here is my

App = Ember.Application.create(); App.ArticleAdapter =DS.FixtureAdapter.extend({});