ActiveModel useful tool that some developers tend to overlook when getting started developing with Rails but that, well used, can add a lot of clarity and convenience to the code in some cases.
The use case of ActiveModel to have a class that behave like an ActiveRecord model, but without DB persistence. That allows, just by importing ActiveModel into a regular Ruby class, to get a lot of useful features that usually belong only to Rails models. These features include validation, serialization, callbacks, attribute methods, etc.
ActiveModel is, somehow, still a bit obscure part of Rails, but the official Rails Guide has some useful examples where we can get a glimpse of how poweful it is.
The story of ActiveModel is interesting as well. Back in 2008 there was an important schism of Rails 2 when Ezra Zygmuntowicz and Yehuda Katz developed [Merb](https://en.wikipedia.org/wiki