Tunr purchases create action

gistfile1.rb

#

  def create
    song = Song.find(params[:song_id])
    if !current_user.songs.include?(song)
      # This line here is failing for some reason.
      # I expect to get true or false, but I'm getting a 
      # nilMethodError.
      if current_user.debit(song.price)
        @purchase = Purchase.create({
          song: song,
          user: current_user,
          price: song.price
        })
      end
    end
    redirect_to songs_path
  end