app.rb

require_relative 'app_config'
require_relative 'models/genre'
require_relative 'models/track'

AppConfig.establish_connection

genres = Genre.where(name: 'Rock')

song = Track.new name: "Genreless Song" # Our Track model says we need to have a genre, so we won't be able to save this.
puts song.save # returns false, since the save failed
puts song.save! # returns an object with all the errors. In this case, we see that Genre is required.

app_config.rb

require 'active_record'
require 'active_support/all'

module AppConfig

  DATABASE = 'chinook'

  DATABASE_PATH = File.absolute_path("#{DATABASE}.db", File.dirname(__FILE__))

  def self.establish_connection
    puts "Connecting to database '#{DATABASE_PATH}'"
    ActiveRecord::Base.establish_connection(
      adapter: 'sqlite3',
      database: DATABASE_PATH
    )
    ActiveRecord::Base.logger = Logger.new(STDOUT)
  end

end

genre.rb

class Genre < ActiveRecord::Base
    has_many :tracks

    def sayHi # there's a method in genre, so we can call <Genre>.sayHi on any Genre instance.
        puts "hello!"
    end

end

track.rb

class Track < ActiveRecord::Base
    belongs_to :genre
    validates :genre, presence: true # validation that says each track must have a genre

    before_validation :make_uppercase # a callback being registered. make_uppercase runs before validation

    after_save :send_email # another callback being registered
  
    def lyrics
        puts "Macarena!"
    end

    private # we put the callback methods inside of a private block, so these methods cannot be called on any Track instance
        def make_uppercase
            puts "MAKING UPPERCASE"
            self.name = self.name.upcase
        end

        def send_email 
            nil # imagine there was some code here to send an email...
        end
end