ActiveRecord::Base.transaction uses this exception to distinguish a deliberate rollback from other exceptional situations. Normally, raising an exception will cause the .transaction method to rollback the database transaction and pass on the exception. But if you raise an ActiveRecord::Rollback exception, then the database transaction will be rolled back, without passing on the exception.
For example, you could do this in your controller to rollback a transaction:
class BooksController < ActionController::Base def create Book.transaction do book = Book.new(params[:book]) book.save! if today_is_friday? # The system must fail on Friday so that our support department # won't be out of job. We silently rollback this transaction # without telling the user. raise ActiveRecord::Rollback, "Call tech support!" end end # ActiveRecord::Rollback is the only exception that won't be passed on # by ActiveRecord::Base.transaction, so this line will still be reached # even on Friday. redirect_to root_url end end