Ruby Advanced Class Methods Lab

Learning Goals

• Build custom class constructors.
• Build class finders.
• Build class operators.

Instructions

This lab has provided you with a base Song class that provides the following definition:

class Song
  attr_accessor :name, :artist_name
  @@all = []

  def self.all
    @@all
  end

  def save
    self.class.all << self
  end

end

The Song class provides a class variable @@all to store all instances for Song that are created through the instance method Song#save. Additionally, Song instances have basic properties of a name and an artist name.

Typographical Convention: Methods we invoke on instances are noted with a #method_name and methods we invoke on classes are noted with a . So Math.sin would refer to the class method sin (divide opposite over adjacent from Trigonometry), Song#save refers to the instance method save that is invoked on an instance of Song:

s = Song.new
s.name = "The Ship Song"
s.artist_name = "Nick Cave"
s.save

You have to build class methods that interact on the class data of @@all and provide the rest of our program with a semantic API on the Song class with methods such as Song.find_or_create_by_name("Blank Space").

Song.create

Build a class constructor Song.create that initializes a song and saves it to the @@all class variable either literally or through the class method Song.all. This method should return the song instance that was initialized and saved.

Consider:

song = Song.create
Song.all.include?(song) #=> true

Song.new_by_name

Build a class constructor Song.new_by_name that takes in the string name of a song and returns a song instance with that name set as its name property. Song.new_by_name should return an instance of Song and not a simple string or anything else. Implement the following functionality:

song = Song.new_by_name("The Middle")
#=> #<Song @name="The Middle">
song.name #=> "The Middle"

Song.create_by_name

Build a class constructor Song.create_by_name that takes in the string name of a song and returns a song instance with that name set as its name property and the song being saved into the @@all class variable.

Consider:

song = Song.create_by_name("The Middle")
#=> #<Song:0x007fd2a2989ff0 @name="The Middle">
song
#=> #<Song:0x007fd2a2989ff0 @name="The Middle">
Song.all.include?(song)
#=> true

Song.find_by_name

Build a class finder Song.find_by_name that accepts the string name of a song and returns the matching instance of the song with that name. Consider:

the_middle = Song.create_by_name("The Middle")
#=> #<Song @name="The Middle">

Song.find_by_name("The Middle")
#<Song @name="The Middle">

Song.find_or_create_by_name

In order to prevent duplicate songs being created that actually represent the same song (based on the song name), we're going to build a Song.find_or_create_by_name class method. This method will accept a string name for a song and either return a matching song instance with that name or create a new song with the name and return the song instance.

Consider:

song_1 = Song.find_or_create_by_name("Blank Space")
song_2 = Song.find_or_create_by_name("Blank Space")

# song_1 and song_2 are conceptually the same song and should return the same song instance because of `.find_or_create_by_name.`

song_1 == song_2 #=> true

Song.alphabetical

Build a class method Song.alphabetical that returns all the songs in ascending (a-z) alphabetical order.

Use Array#sort_by.

Song.new_from_filename

Build a class constructor that accepts a filename in the format of " - .mp3", for example, "Taylor Swift - Blank Space.mp3".

Given Song.new_from_filename("Taylor Swift - Blank Space.mp3"), the constructor should return a new Song instance with the song name set to Blank Space and the artist_name set to Taylor Swift. The filename input sent to Song.new_from_filename in the format of Taylor Swift - Blank Space.mp3 must be parsed for the relevant components. Separate the artist name from the rest of the data based on the - delimiter. Don't forget that when you parse the song name, you have to remove the '.mp3' part of the string.

song = Song.new_from_filename("Taylor Swift - Blank Space.mp3")
song.name #=> "Blank Space"
song.artist_name #=> "Taylor Swift"

Song.create_from_filename

Build a class constructor that accepts a filename in the format of " - .mp3", for example "Taylor Swift - Blank Space.mp3". The Song.create_from_filename class method should not only parse the filename correctly but should also save the Song instance that was created.

Song.destroy_all

The Song.destroy_all class method should reset the state of the @@all class variable to an empty array thereby deleting all previous song instances.