Nikola pointed out ActiveWarehouse , a new RoR Data Warehousing project. Haven’t tried it out myself but the author has posted an excellent tutorial (post deleted but can be found here on Wayback Machine) on using the plug-in to create an example warehouse. Along with its ETL and SQL Views sub-projects ActiveWarehouse provides a great foundation for Ruby based BI systems. Being a early stage RoR project the use of MySQL as the database layer is assumed but as it’s based on ActiveRecord adding support for other relational databases should be trivial.
Along side ROLAP support, the ability to load and retrieve data from MOLAP sources such as the open source Palo server would be a nice addition; while the ability to combine both MOLAP and ROLAP together under one platform (as Oracle now supports) would be fantastic.