def facebook
      # You need to implement the method below in your model (e.g.
app/models/user.rb)
      @user = User.from_omniauth(request.env["omniauth.auth"])

      if @user.persisted? ←★なぜfalseになってしまうのか？★
        sign_in_and_redirect @user, :event => :authentication #this will throw if @user is not activated
        set_flash_message(:notice, :success, :kind => "Facebook") if
is_navigational_format?
      else ←★すべて条件分岐先がelseになってしまう。★
        session["devise.facebook_data"] = request.env["omniauth.auth"]
        redirect_to new_user_registration_url
      end
  end

def self.from_omniauth(auth)
    where(provider: auth.provider, uid: auth.uid).first_or_create do |user|
      user.email = auth.info.email
      user.password = Devise.friendly_token[0,20]
    end
end