Ok so here is my situation. Im a PHP developer recently turned Rails developer. In PHP code the use of abstract classes as "base" classes is a pretty common pattern. When moving onto my current Rails application I did a lot of the "pre-planning" with PHP ideas in my head which are now giving me a bit of a headache.
I have many different types of Invoices. In PHP I would create an abstract class and have my different types of Invoices that extend that class. I would DRY things up by keeping duplicate code in the abstract class and life is good. In rails things work a bit differently.
I can't seem to find a pattern that fits perfectly to my preconceptions. STI is nice but I HATE the idea of all the Null fields since each invoice will have many different fields. I have come close with Polymorphic associations using a module to DRY up my code. However, everywhere I look polymorphic associations are used differently then the way I intend. For example, in the railscast there are three different classes that each need to use comments. Its not exactly the same thing.
So here is my question: I already know how to use a module to DRY up my code, but how can I use polymorphic associations to provide similar modularity to my code as the "base" classes do in php? FYI I can't use STI because there is no limit on how many different types of invoices could end up coming into existence. For now its 1, but in a month it could be 20.
