restructuring tables and model relationships in order to use STI

Hey James! I'm going back to the garage app and want to implement some of the things I learned from you over the last couple of weeks. As a first step, I'm restructuring the models and databases. Planning to use STI for Users (with Customers and Employees inheriting from User) and for Repairs, with Invoices and Estimates inheriting from Repairs.
You're right! Blanked on this one. Will fix.