NPR Story Puller with some OO

downloader.rb

class Downloader
  attr_accessor :stories, :selection

  def initialize(selection = nil, stories)
    self.stories = stories
    self.selection = selection
  end

  def current_story
    stories[selection - 1]
  end

  def exists?
    selection > 0 && selection < stories.length
  end

  def exit?
    selection == 0
  end

  def to_s
    str = ''
    stories.each_with_index do |story, index|
      str += "#{index + 1}. #{story.title}\n"
    end
    str
  end
end

main.rb

require './npr_retriever'
require './story'
require './downloader'

puts "Welcome to the NPR Music Latest & Greatest\n\nThis program shows the last 20 stories on NPR Music with audio available for download, and lets you quickly download any of them.\n\nHere are the 20 most recent stories:"

downloader = Downloader.new(nil, Story.all)
puts downloader.to_s

while !downloader.exit?
  puts "\nEnter the number of the audio file you'd like to download, or enter 0 to leave the program."
  downloader.selection = gets.chomp.to_i

  if downloader.exists?
    system('open', downloader.current_story.audio_file)
  elsif downloader.exit?
    puts "Thanks for using NPR Music Latest & Greatest"
  else
    puts "\nSorry, that's not a valid selection."
  end
end

npr_retriever.rb

require 'rest-client'

class NPRRetriever
  API_KEY = 'MDE4ODc1MDMwMDE0MjkxNDI3MDc0NTgzNg001'
  API_URL = "http://api.npr.org/query?id=10001&fields=title,teaser,audio,artist&requiredAssets=audio&dateType=story&sort=dateDesc&output=JSON&numResults=20&apiKey=#{API_KEY}"

  def self.get_stories
    JSON.parse(RestClient.get(API_URL))['list']['story']
  end
end

story.rb

class Story
  attr_accessor :title, :audio_file

  def initialize(attrs)
    self.title = attrs['title']['$text']
    self.audio_file = attrs['audio'].first['format']['mp3'].first['$text']
  end

  def self.all
    stories = []
    NPRRetriever.get_stories.map do |full_story|
      stories << Story.new(full_story)
    end
    stories
  end
end