Lafcadio is an object-relational mapping library for Ruby. It currently supports MySQL and PostgreSQL. Its design has a few aspects in mind:

• The importance of unit-testing. Lafcadio includes a MockObjectStore which can take the place of the ObjectStore for unit tests, so you can test complex database-driven logic. Committing domain objects, running queries, and even triggers can all be written in the Lafcadio level, meaning that they can all be tested without hitting a live database.
• Dealing with databases in the wild. Lafcadio excels at grappling with pre-existing database schemas and all the odd ways the people use databases in the wild. It requires very little from your schema, except for the fact that each table needs a single numeric primary key. It makes many assumptions about your naming conventions, but these assumptions can all be overridden.

First-time users are recommended to read the tutorial at lafcadio.rubyforge.org/tutorial.html.