Strategy pattern files location Rails

I have a couple of questions: following the perfect response that I got on that question Business logic dependent on model attribute.

I plan on usin a Strategy pattern, and I was wondering where I (in which folder) should put the classes files? should they go in the models folder? Also, I have a default behavior, do I still need to create a Interface class and have my default strategy derived from this, or I define my default strategy as the base strategy?