Rails - Counter Cache

Counter Cache

https://rubyinrails.com/2019/04/10/rails-counter-cache-associations/

https://scoutapm.com/blog/how-to-start-using-counter-caches-in-rails

For many-many association:

https://coderwall.com/p/muvy9w/forget-counter-cache-for-many-to-many-associations-in-rails

https://stackoverflow.com/questions/9595338/counter-cache-for-a-model-with-a-many-to-many-association

https://stackoverflow.com/questions/5256897/counter-cache-with-has-many-through