Ruby on Rails Book question

Wondering if anyone is familiar with this part of the book. It’s in
Part IV , the enhancements to the R4RMusic site, chapter 15, soft and
hard enhancements.
Following along it shows various methods being defined for work.rb
(works model)
However then it comes to a ‘def open_orders’. Trying to figure out if
this method is intended for work.rb or another model file. Not really
clear. Seems more like an order.rb method.