如何在同一个Ruby Rails项目中配置MongoMapper和ActiveRecord

I've got an existing production Ruby/Rails app that I want to migrate to MongoDB over time, as time permits, because it's not an option to just rewrite it all at one time. I'd love to be able to just deprecate my old classes as I get to them. I plan to use MongoMapper. I can't find an example where anyone explains how to set up the database config files to allow connection to both data stores within one app.I've got an existing production Ruby/Rails app