Question

Hi I’m currently going through a tutorial on unit tests. I have two models: Song and Album.

class Song < ActiveRecord::Base
  attr_accessible :title, :duration_in_seconds
  belongs_to :album
end

class Album < ActiveRecord::Base
  attr_accessible :artist, :title

  has_many :songs


  def duration
    songs.sum(&:duration_in_seconds)
  end

end

This is the test that’s supposed to pass:

require 'test_helper'

class AlbumTest < ActiveSupport::TestCase
  test "should be able to report duration based on combined duration of its songs" do
    album = Album.create
    3.times do
      album.songs.create(:duration_in_seconds => 5)
    end
    assert_equal 15, album.duration
  end
end

As you see in the Album model, there is a method called duration. I have two questions about that.

1. So we know that Song belongs to Album. If I do a method on an instance of Album, I can just access the songs of that instance in that method directly like that? The method definition looks weird… @album.duration would return @album.songs.sum(&:duration_in_seconds)? Lets say I had this method in my Album method:

def name_of_album

title

end

If I called @album.name_of_album would it be the same as @album.title?

1. What’s with the & in the duration method? Is it the same as songs.sum(:duration_in_seconds)?

Answer 1

The ampersand is part of Ruby and when it’s used in front of a symbol, it converts it to a Proc.
See http://railscasts.com/episodes/6-shortcut-blocks-with-symbol-to-proc?view=asciicast for more details.