First of all, this book is really awesome.
I got a problem on the downmethod ( source:https://github.com/addisaden/learning-rails/commit/33eea1ffad963cdcbc4443266d3bfd5ae9940dfa ) on page 122.
When i try to Rollback, it rolls back but the rails app doesnt work any more. it shows ActiveRecord::PendingMigrationError. After rake db:migrate it runs again.
I also researched the cart after the Rollback via rails console and the items showed up with a quantity of 1. So for me its not possible to solve this, because i am new in rails :)
Thanks a lot Adrian
edit: after cloning the project from github on a clean directory i migrate all, add a ruby book, rollback and get the same issue.