You should have a base model for all your models where you put shared logic and interfaces for all other models. There is a whole lot of repetition in your code, in models like Genre and Artist you keep including and extending modules some of this method in this module can be kept in your base model.

Also, I want you to research on when to use mixin vs inheritance, I will be glad if you share your findings with me. Thanks.

Try as much as possible to not overdo UI, in your case, the UI is affecting the test and also some portion affects the UX, making it take longer than expected. In most cases a simple UI should suffice.

Your UI & UX should revolve around the goal of your application. In this case, an app that makes it easy for users to find and play music in their library. Overdoing it is an overkill. When building UI & UX keep this maxim in mind (Keep it Simple)

I am not asking you to change your UI but just take note of this in the case of next time.

Next time, adhere to git best practices, I see you only have a master branch. In future scenarios, create a different branch for each feature you are to work on.

You should stay away from using reserved keywords. If you take a look at here you have a method called put define on line 76 and puts is also called on line 10 tho, they are different but it creates mystery guess There are several instances of this same puts method in this file, instead, rename the method to what seems appropriate. To me, this looks like a nice hack

Take advantage of folders to structure your code. At this point, it may seem trivial but it's very important as a developer. In your lib folder, leaving code laying on different aspect without grouping them is really not a good approach. So go create models folder, and controllers folder move all your models to the model folder and your music_library_controller.rb file to your controller folder. This gives your code a good folder structure. Ensure you change the path to require in your music_library_controller.rb file to make it work.