DataMapper is a storage engine agnostic ORM, it can work just as well with a RDMS as with any another type of storage engine such as, but not limited to: no sequel stores (redis, mongo...), XML or yaml data source, etc. While many adapters already exist to connect DataMapper to a miriad of storage engines, there may not be one for your preferred storage engine or you may need to communicate with data from a legacy system. If this is the case I will introduce you to the clean and simple API used to write your custom DataMapper adapter.
I am a ruby/rails developer working for OptimisCorp in Taiwan. I haven been involved with ruby, personally and professionally, for over 2 years. I tend to enjoy deploying to the cloud and have spent a lot of time working with Ruby on the Google App Engine. You can find more about me at github: https://github.com/joshsmoore.