# 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.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 #=> # 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.find_by_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](http://ruby-doc.org/core/Enumerable.html#method-i-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.