In the previous unit, we used validations and forms to create a management dashboard for songs and artists. Now we're ready to start connecting those models and using their relationships to display more interesting data.

• In the controllers, this lab will require you to write show and index actions for both Song and Artist.

• You'll need to add a foreign key by writing your own migration.

• You'll need to set up the association in the model classes.

• You'll need to format and link to the songs and artists according to the specs.

Additionally, you'll find that the specs require two special methods, Artist#song_count and Song#artist_name. These methods are meant to protect the views from complexity that belongs in the model.

They are also a great example of a software design principle called the Law of Demeter, which is sometimes called the "one dot" rule in object-oriented languages like Ruby. In other words, ruby user.best_friend is better than ruby user.friends.find_by(best: true) because all of the "friend"-related complexity is hidden away within the user model. This protects user-related code from future changes to friend functionality.

For example, if the above architecture changed such that best friendship was determined by the highest "friendship" value instead of a boolean best flag, the "two-dots" code would need to be changed everywhere, but the first snippet, which obeys the Law of Demeter, hides that complexity in the User#best_friend method, whose definition can be changed without having to track down and update every single usage.

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