NameError: uninitialized constant Article::ImageUploader when using Carrierwave on rails 4.1.5

I'm learning rails 4.1.5 I got this error: 2.1.1 :008 > Article NameError: uninitialized constant Article::ImageUploader from /Volumes/disk0s4/www/rails/blog/app/models/article.rb:4:in `<