Can you create a ruby api application on top of existing schema?

I am fairly new to Ruby on Rails. I am using it to create a web API application and was wondering if instead of creating a schema based on my Model, can I do the reverse? E.g. is it possible to create model that would fit with an already existing schema? Something like that would be fairly easy in Java World using JPA but I am not so sure about Rails using DSL for databases.

Do I have to manually change the migration files in this case? If yes, is there an easy/recommended way to do this?

Thanks