redirect_to :backをなんとなく使ってみたら DEPRECATION WARNING: redirect_to :back is deprecated and will be removed from Rails 5.1. Please use redirect_back(fallback_location: fallback_location) where fallback_location represents the location to use if the request has no HTTP referer information. ..とおこられてしまった. つまり request.refererがnilなとき,fallback_locationにリダイレクトしてくれる ってことなんでしょうが、なんでそもそもredirect_to :backが非推奨なのか少