Before

class OmniauthCallbacksController < Devise::OmniauthCallbacksController

  def facebook
    user = User.from_omniauth(request.env["omniauth.auth"])
    if user.persisted?
      set_flash_message(:notice, :success, kind: "Facebook") if is_navigational_format?
      sign_in_and_redirect user, event: :authentication
    else
      session["devise.user_attributes"] = user.attributes
      redirect_to new_user_registration_url
    end
  end

  def twitter
    user = User.from_omniauth(request.env["omniauth.auth"])
    if user.persisted?
      set_flash_message(:notice, :success, kind: "Twitter") if is_navigational_format?
      sign_in_and_redirect user, event: :authentication
    else
      session["devise.user_attributes"] = user.attributes
      redirect_to new_user_registration_url
    end
  end
end

After


class OmniauthCallbacksController < Devise::OmniauthCallbacksController

  def callback_for_all_providers
    provider = __callee__.to_s  # エイリアスメソッド名を読みこみ、文字列として利用。

    user = User.from_omniauth(request.env["omniauth.auth"])
    if user.persisted?
      sign_in_and_redirect user, event: :authentication
      set_flash_message(:notice, :success, kind: provider.capitalize) if is_navigational_format?
    else
      session["devise.user_attributes"] = user.attributes
      redirect_to new_user_registration_url
    end
  end

  alias_method :twitter, :callback_for_all_providers
  alias_method :facebook, :callback_for_all_providers
end