Transactional “Reserve Inventory” with SELECT … FOR UPDATE

                  class Inventory::Reserve
  class NotEnoughStock < StandardError; end

  def initialize(sku:, quantity:)
    @sku = sku
    @quantity = Integer(quantity)
  end

  def call
    ApplicationRecord.transaction do
      item = InventoryItem.lock.find_by!(sku: @sku)
      raise NotEnoughStock if item.available < @quantity

      item.update!(available: item.available - @quantity)
      InventoryReservation.create!(sku: @sku, quantity: @quantity)
    end
  end
end