Anthony Eden has a project for this from a while back: http://activewarehouse.rubyforge.org/ Not sure how well its maintained at this point but it looks like a pretty good mapping of the concepts to me.
my bad..my english is not good..what i meant to ask was ORM needs Object Models of data schema..And its easy to make from relational database perspective..but when it comes to star schema perspective, what can be the trade offs?