While we went in to CRUDing in depth with a single model, and we explored what it means to link, or relate, our models, we haven't shown you an example of some of the tools Rails gives us to CRUD related models.
The Rails Guides for ActiveRecord Associations has great information on how to do this! You need to read them! But they don't walk you through each step of implementing them in an app.
To that end, we have four examples/walkthroughs to show you about how to structure the models, views, controllers and routes when trying to CRUD resources whose structure depends upon their relations: