[Update: Rebuilding Rails is finished, and yes, the exercises are in it.]
A reader asked me whether the book was entirely cut and paste or if there would be exercises that required actually writing new code in Rebuilding Rails, or only cut-and-paste. Here’s my response:
There are a few small exercises in chapter one, and will be more and more extensive ones later. However, I’m not sure if it will be extensive enough for what you want.
For instance, in the new ORM chapter there will be an exercise on adding column accessors via method_missing, which requires a good basic grasp of what’s going on.
There will be 3-5 exercises of about that difficulty (many a bit easier, a few significantly harder) in each chapter. The book is structured so that your framework gets better with each exercise, but no later chapter depends on the exercise being finished. So if you skip them, you get the pure cut-and-paste experience but it still works. If you do all of them, you’ll have a much more usable framework.
The downloadable sample code just gives the base cut-and-paste framework, not the exercises.