Devise gem ; email を sign up に使わない方法は

先ず無い

引用元

https://github.com/plataformatec/devise/wiki/How-To%3a-Allow-users-to-sign-in-with-something-other-than-their-email-address

Important note: remember that even if you don’t want to sign in using email, you must have email in your sign up form. See StackOverflow thread for more information. If you don’t want to require an email address, simply define an email_required? function in your model, and have it return false.

gem 自体を変えずにやれないことはなさそうだが、時間がかかる。

Obey master.