Music Library CLI

Overview

You're going to be implementing a Music Library domain composed of 3 main models, Song, Artist, and Genre. The models will relate to each other and collaborate heavily. Additionally, you're going to be extracting some common functionality out of those models and into a module, Concerns::Findable, which you'll then mix back into the models. You'll then build a collaborating object, MusicImporter, that can parse a directory of MP3 files and use the extracted filenames to create instances of Song, Artist, and Genre objects. Finally, you'll build a CLI in bin/musiclibrary that is powered by a MusicLibraryController to provide a simple CLI that lets a user browse the library of MP3s imported by song, artist, and genre.

This is a complex lab with many parts, so go slowly. Take time to understand what you're building holistically before starting. Read this entire README before jumping in. As you go from spec to spec, we recommend doing them in numbered order.

Instructions

Song, Artist, and Genre basics

The first thing to do is get the basics of the main models working. Each model has almost the exact same basic requirements, so once you make 001_song_basics_spec.rb pass by building the Song class, the basic Artist and Genre specs will go quickly.

The requirements for each model are that they can accept a name upon initialization and set that property correctly. The name property should be readable and writable by the object.

Song.new("Blank Space").name #=> "Blank Space"`

Additionally, each class should contain a class variable @@all that is set to an empty array and is prepared to store all saved instances of the class. This class variable should be accessible via the class method .all.

Song.all #=> []

Artist.all #=> []

Instances should respond to a #save method that adds the instance itself into the appropriate @@all class variable.

Song.new("Blank Space").save
Song.all #=> [#<Song: @name="Blank Space">]

The class should be able to empty its @@all array via a class method .destroy_all.

Song.new("Kaohsiung Christmas").save
Song.all #=> [#<Song: @name="Blank Space">, #<Song: @name="Kaohsiung Christmas">]
Song.destroy_all
Song.all #=> []

Finally, all classes should implement a custom constructor .create that instantiates an instance using .new but also invokes #save on that instance, forcing it to persist immediately.

Song.new("Blank Space")
Song.all #=> []
Song.create("Blank Space")
Song.all #=> [#<Song: @name="Blank Space">]

Relationships

Songs and Artists

• Songs belong to an artist and an artist has many songs. Adding a song to an artist is done by calling an #add_song method on an instance of the Artist class.

• Songs can be initialized with an optional artist argument.

Songs and Genres

• Genres have many songs and are initialized with an empty list of songs.

• Songs have one genre.

• Songs can be initialized with an optional genre argument.

Artists and Genres

• Artists have many genres through their songs. Implement a #genres method for this association.

• Genres have many artists through their songs. Implement an #artists method for this association.

Note: there are a few tests concerned with switching the Song#initialize method from setting instance variables for @artist and @genre to using the custom setter methods that you define (e.g., Song#genre=). We want to use the custom setter methods because they keep our associations in sync. For example, when we call our custom Song#artist= method, it sets the song's @artist property and adds the song to the artist's collection of songs. When you reach these tests, make sure those setter methods are only invoked if Song#initialize is called with artist and/or genre arguments. Otherwise, the @artist and/or @genre properties will be initialized as nil, and you'll have some unexpected consequences in both your code and the test suite.

• If we call Song.new("Song Title", artist_object, genre_object), both Song#artist= and Song#genre= should be invoked.

• If we call Song.new("This Song Has No Artist or Genre"), neither Song#artist= nor Song#genre= should be invoked.

Finding

Song

First implement the following two methods in your Song class:

• Songs should have a find_by_name method.

• Songs should have a find_or_create_by_name method.

Concerns::Findable

Now that you've gotten the methods working in Song, let's adapt them for general reuse by putting them into a module that we can mix into our Artist and Genre classes. It's Ruby convention to put modules in a concerns/ folder nested under lib/, and each module should be namespaced like this:

module Concerns::ModuleName
  # Module code here
end

Once the basic module structure is good to go, it's time to code our two class methods again:

• Implement a generic #find_by_name method that uses the .all method defined by the class to find an instance of the class by name.

• Implement a generic #find_or_create_by_name method that uses the .all method defined by the class to find an instance of the class by name and to create a new instance if a match is not found.

• Add this module to your Genre and Artist class.

MusicImporter

Create a MusicImporter class that works with your Song, Genre, and Artist objects to import a directory of MP3 files. This class will have the following methods:

• #initialize accepts a file path to a directory of MP3 files.

• #files returns all of the imported filenames.

• .import imports all of the files from the library, instantiating a new Song object for each file.

In addition, add the following pair of methods to your Song class:

• .new_from_filename, which instantiates a new Song object based on a provided filename.

• .create_from_filename, which does the same thing as .new_from_filename but also saves the newly-created song to the @@all class variable.

It's CLI time!

Congrats! You've done the heavy lifting. Now let's wrap it all up in a simple CLI so that users can actually interact with our code. Create a MusicLibraryController class that:

• Upon initialization, accepts an optional path to the library of MP3 files, defaulting to ./db/mp3s/. It should then instantiate a MusicImporter object, which it will use to import songs from the specified library.

• Has a #call method that starts the CLI and prompts the user for input. Read the tests carefully for specifics.

Have fun!

Resources

• QA with Students

  • This Q&A led by Avi Flombaum covers setting up a bin file, setting up a Gemfile and installing gems, and identifying objects and their responsibilities. It contains general tips on requirements, gems, and design.

• Debugging an Error in Music Library CLI

  • This walk-through traces an error to its source in the code. In addition it covers how to change file permissions and how to create an executable file to initialize a sandbox environment.

• Lab Review

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