[ERROR] "Session contains objects whose class definition isn't available. Remember to require the classes for all objects kept in the session"

エラーメッセージ

Session contains objects whose class definition isn't available.
Remember to require the classes for all objects kept in the session.
(Original exception: uninitialized constant MySQL [NameError])

対応

古いセッション（ブラウザのクッキー）を削除.

参考サイト

RAILS :“session contains objects whose class definition isn\'t available.”
セッションから復元できないモデルオブジェクトに対処する