For a variety of reasons, you might find yourself supporting multiple databases in your Rails application. Maybe you're connecting to a legacy database for a few models. Perhaps you have divided your Rails application into two parts, one database for your online catalog system and another for transactional data. Multiple database connections in Rails is nothing new. While there may be great benefits to connecting to multiple databases in your app, there are also costs. One example is that has_many :children, :through => parent_children may not work.