def by_order_production_deadline_then_customer(list)
  by_deadline = sort_order(:production_deadline)
  by_customer = sort_order(:customer_name)

  list.sort do |a, b|
    primary_sort_order = by_deadline[a, b]
    secondary_sort_order = by_customer[a, b]

    if primary_sort_order != 0
      primary_sort_order
    else
      secondary_sort_order
    end
  end
end