Avid's book: http://objectsonrails.com/#ID-4f5368c3-33f8-4172-ac6b-572b6ca0bc22
Avid's book: http://objectsonrails.com/#ID-f30ba520-fb59-4d77-b30e-4e862cdb37fe
Avid's book: http://objectsonrails.com/#ID-3ab4efe2-3f1a-47fc-b5c7-79e22e404d30