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# Has Many Objects Through Lab

## Objectives

* 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.

## Overview

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. `Appointment`s belong to a `Patient` and a `Doctor`. A `Doctor` has many `Appointment`s and a `Patient` has many `Appointment`s. A `Doctor` has many `Patient`s through its `Appointment`s and a `Patient` has many `Doctor`s through its `Appointment`s.

## Instructions

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.

**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 `Appointment`s and finds those belonging to this doctor.
* The `Doctor` class needs an instance method, `#patients`, that iterates over that doctor's `Appointment`s and collects the patient that belongs to each `Appointment`s.

```
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 `Appointment`s array and returns `Appointment`s that belong to the patient.
* The `Patient` class needs an instance method, `#doctors`, that iterates over that patient's `Appointment`s 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)
```

View [Has Many Objects Through Lab](https://github.com/learn-co-curriculum/ruby-objects-has-many-through-lab) on Learn.co and start learning to code for free.