• Write classes that are related to each other via the "has-many-through" relationship.
• Write methods that use the "has-many-through" relationship to deliver information on related objects.

In this lab, we'll be dealing with two sets of three classes.

We have a Song, Artist and Genre class. Songs belong to an artist and belong to a genre. A genre has many songs and an artist has many songs. An artist has many genres through its songs and a genre has many artists through its songs.

We also have an Appointment, Doctor and Patient class. Appointments belong to a Patient and a Doctor. A Doctor has many Appointments and a Patient has many Appointments. A Doctor has many Patients through its Appointments and a Patient has many Doctors through its Appointments.

Start with the Artist/Song/Genre domain.

The Artist model:

• The Artist class needs a class variable @@all that begins as an empty array
• The Artist class needs a class method .all that lists each artist in the class variable
• An artist is initialized with a name and is saved in the @@all array.
• The Artist class needs an instance method, #new_song, that takes in an argument of a name and genre creates a new song. That song should know that it belongs to the artist.
• The Artist class needs an instance method, #songs, that iterates through all songs and finds the songs that belong to that artist. Try using select to achieve this.
• The Artist class needs an instance method, #genres that iterates over that artist's songs and collects the genre of each song.

madonna = Artist.new("Madonna")

The Genre model:

• The Genre class needs a class variable @@all that begins as an empty array.
• The Genre class needs a class method .all that lists each genre in the class variable.
• A genre should be initialized with a name and be saved in the @@all array.
• The Genre class needs an instance method, #songs, that iterates through all songs and finds the songs that belong to that genre.
• The Genre class needs an instance method, #artists, that iterates over the genre's collection of songs and collects the artist that owns each song.

Now let's move on to our Doctor/Appointment/Patient domain model.

pop = Genre.new("pop")

The Song model:

• The Song class needs a class variable @@all that begins as an empty array.
• The Song class needs a class method .all that lists each song in the class variable.
• A song should be initialized with a name, an artist, and a genre, and be saved in the @@all array.

madonna = Artist.new("Madonna")
pop = Genre.new("pop")

into_the_groove = Song.new("Into the Groove", madonna, pop)

The Doctor model:

• The Doctor class needs a class variable @@all that begins as an empty array.
• The Doctor class needs a class method .all that lists each doctor in the class variable.
• A doctor should be initialized with a name and be saved in the @@all array.
• The Doctor class needs an instance method, #new_appointment, that takes in a an instance of the Patient class and a date, and creates a new Appointment. That Appointment should know that it belongs to the doctor
• The Doctor class needs an instance method, #appointments, that iterates through all Appointments and finds those belonging to this doctor.
• The Doctor class needs an instance method, #patients, that iterates over that doctor's Appointments and collects the patient that belongs to each Appointments.

doogie = Doctor.new("Doogie Howser")

The Patient model:

• The Patient class needs a class variable @@all that begins as an empty array.
• The Patient class needs a class method .all that lists each patient in the class variable.
• A patient is instantiated with a name and be saved in the @@all array.
• The Patient class needs an instance method, #new_appointment, that takes in an argument of a doctor and a date and creates a new Appointment. The Appointment should know that it belongs to the patient.
• The Patient class needs an instance method, #appointments, that iterates through the Appointments array and returns Appointments that belong to the patient.
• The Patient class needs an instance method, #doctors, that iterates over that patient's Appointments and collects the doctor that belongs to each Appointment.

ferris = Patient.new("Ferris Bueller")

The Appointment model:

• The Appointment class needs a class variable @@all that begins as an empty array.
• The Appointment class needs a class method .all that lists each Appointment in the class variable.
• An Appointment should be initialized with a date (as a string, like "Monday, August 1st"), a patient, and a doctor. The Appointment should be saved in the @@all array.

ferris = Patient.new("Ferris Bueller")
doogie = Doctor.new("Doogie Howser")

new_appointment = Appointment.new('January 1st, 1989', ferris, doogie)

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