DataMapper is an object-relational mapper library written in Ruby that follows the active record pattern even though the name implies it follows the data mapper pattern. While DataMapper 1 may not have achieved total decoupling between object and database suggested by the data mapper pattern, it appears DataMapper 2 intended to change this (a la Virtus, a library adapted from DataMapper). The DataMapper 2 project was renamed before launch and was released as Ruby Object Mapper (ROM) in August 2013.
Some features of DataMapper:
DataMapper was designed to be a more abstract ORM, not strictly SQL, based on Martin Fowler's enterprise pattern. As a result, DataMapper adapters have been built for other non-SQL databases, such as CouchDB, Apache Solr, and webservices such as Salesforce.com.