Transaction fixtures don't work if model has a different connection than ActiveRecord::Base

When using transactional fixtures each example is wrapped in a database transactions. This transaction is started on ActiveRecord::Base's connection. However if you use fixture for a model that has a different connection, any changes to this model are not inside of a transaction. This means that changes to this table accumulate and are not reset between examples.

The solution should be fairly simple. Instead of only using database transaction on AR::Base, start a transaction against every connection referred to by a model.

attached is a diff based on Rails 2.2.2 which illustrates the basic idea. I'll create a proper patch against HEAD shortly, as well as write some test.