# lib/transaction_filter.rb
class TransactionFilter
  def filter(controller)
    return yield if controller.request.get?
    ActiveRecord::Base.transaction do
      yield
    end
  end
end

# application.rb
class ApplicationController < ActionController::Base
  around_filter :transaction_filter
  def transaction_filter(&block)
    TransactionFilter.new.filter(self, &block)
  end
end