Note: Before diving into this implementation, consider that there are more popular and potentially easier-to-use solutions for implementing Sign in with Apple in Ruby on Rails, such as Omniauth. If you prefer a more out-of-the-box approach, explore those alternatives.
This implementation of the Sign in with Apple service in Ruby on Rails is suitable for APIs, eliminating the need for views.
This is another piece of code I've extrapolated from a Ruby on Rails project I'm currently working on. The code implmenets social login with a RoR API-based application, targeted at API clients.
The setup does not involve any browser-redirects or sessions as you would have to use working with Omniauth.
Instead, what it does is takes an access_token generated on client-side SDKs, retireves user info from the access token
and creates a new user and Identity in the database.
This setup works with native applications as described in the Google iOS Sign In Docs (see Authenticating with a backend server)