class Album < ActiveRecord::Base
  has_many :songs
  belongs_to :artist
  has_many :transactions
end

class Artist < ActiveRecord::Base
  has_many :albums
  has_many :transactions
end

class Song < ActiveRecord::Base
  belongs_to :album
  has_many :transactions
end

class User < ActiveRecord::Base
  has_many :transactions
end

class Transaction < ActiveRecord::Base
  belongs_to :user
  
  belongs_to :album
  belongs_to :song
  belongs_to :artist
end