最終話 https://guides.rubyonrails.org/getting_started.html#extracting-a-concern Concerns are a great way to organize features of your Rails application. As you add more features to the Product, the class will become messy. Instead, we can use Concerns to extract each feature out into a self-contained module like Product::Notifications which contains all the functionality for handling subscribers and how notifications are sent. Extracting code into concerns also helps make features reusable. For example, we could introduce a new model that also needs subscriber notifications. This module could be used in multiple models to provide the same functionality.