to_excel.rb

    def column_grades(program_assignment, answer_sets)
      CSV.generate(col_sep: "\t") do |csv|
        problem_set_headings = program_assignment.problem_sets.published.order(:due).map(&:name)
        csv << ["name", "student id", "student email"] + problem_set_headings
        answer_sets.each_pair do |user, ans_sets|
          grades = {}
          ans_sets.map{|as| grades[as.problem_set_id] = as.grade }
          user = ans_sets.first.user
          row = [user.name, user.studentid, user.email]
          program_assignment.problem_sets.published.order(:due).each do |ps|
            if grades[ps.id].present?
              row << grades[ps.id].round(3)
            else
              row << 0
            end
          end
          csv << row
        end
      end
    end