Rails ActiveRecord Associations - Join Tables (Many-to-Many), Polymorphic

https://roseweixel.github.io/blog/2014/10/22/join-tables-in-active-record/ 

https://medium.com/@pk60905/many-to-many-relationship-in-rails-cf86e12db8b0

https://medium.com/@pk60905/polymorphic-association-4deb6f87bf8