最近rails3.2在更改数据库表字段,然后要回滚取消,但在运行rake db:rollback命令,错误: rake aborted! An error has occurred, all later migrations canceled: ActiveRecord::IrreversibleMigration/usr/local/rvm/gems/ruby-1.9.3-p392/gems/activerecord-3.2.14/lib/active_record/migration/com