I'm abstracting the payment gateway I'm using out a a Subscription class and into a PaymentGatewayApi class so that it can be easily changed in the future without touching models. My APIs are located in `app/api` which appears to be just fine as I only have a few of them (although I'll move them if someone smarter than me says I should).

In trying to write quality code and reducing my parameter calls, I need to make some simple utility classes. One of these is a `CreditCardCustomer` that can be instantiated in the `Subscription` model and then have the instance passed to the `PaymentGatewayApi`.

My question is, where should I put this class? Does it need to be in its own file, or can I put multiple classes in the same file? What is the best practice both for Ruby in general and Rails in particular?