ephebe

假設系統有一個Order、OrderDetail、ProductDesc的類別關係如下

因為需求增加，所以Order在結帳時可以使用信用卡消費，但必須加收一成服務費。為此在Order加開欄位IsCreditCard，然後來修改程式，判斷是否為信用卡消費。

class Order
{
       public void ReCalcualte
       {
           if(IsCreditCard == false)
              SumAmount = 所有OrderDetail的Price * Unit
           else
              SumAmount = 所有OrderDetail的Price * Unit * 1.1
       }
}

假設將來結帳方式增加了，bit欄位不夠用，就會變成int或varchar，ReCalcualte也會更加複雜，所以我用繼承來試著處理這個問題。 NH中處理繼承的手法很多，就先從Table Per Class著手。首先我想把信用卡消費分開成一個CreditCardOrder，CreditCardOrder內容如下

public class CreditCardOrder : Order
{
        public virtual decimal DiscountAmount { get; set; }

        public override void ReCalculate()
        {
            base.ReCalculate();

            SumAmount = SumAmount + SumAmount*0.1m;

            DiscountAmount = SumAmount*0.1m;
        }
}

Order的Mapping的設定如下

所謂的Table Per Class是運用一個欄位的值做判斷，決定子類別的類型，這個例子中，discriminator中指定了IsCreditCard這個欄 位，discriminator-value="Y"告訴我們如果欄位是Y，會回傳CreditCardOrder物件，若為null，則會傳Order 物件。discriminator除了string型別外，也可以指定int，可是卻不能為null，也就是每列都對應一種子類別。
現在各建立一個Order物件與一個CreditCardOrder物件

    Order order = new Order();
    order.ShoppingDate = new DateTime(2011, 7, 14);

    ProductDesc productDesc = new ProductDesc();
    productDesc.Name = "原子筆";
    productDesc.Price = 13;

    OrderDetail orderDetail = new OrderDetail();
    orderDetail.Quantity = 20;
    orderDetail.SellPrice = 10;
    orderDetail.SellProductDesc = productDesc;

    order.AddDetail(orderDetail);
--------------------------------------------------------------
     CreditCardOrder creditCardOrder = new CreditCardOrder();
     creditCardOrder.ShoppingDate = new DateTime(2011, 7, 14);

     ProductDesc productDesc = new ProductDesc();
     productDesc.Name = "鉛筆";
     productDesc.Price = 8;

     OrderDetail orderDetail = new OrderDetail();
     orderDetail.Quantity = 20;
     orderDetail.SellPrice = 10;
     orderDetail.SellProductDesc = productDesc;

     creditCardOrder.AddDetail(orderDetail);

來看看查詢的結果

    IList<[order]> orders = session.CreateCriteria<[order]>().List<[order]>();

    //雖然是查Order，但連CreditCardOrder也會查出
    foreach (Order order in orders)
    {
        order.ReCalculate();
    }

    Assert.AreEqual(200, orders[0].SumAmount);
    Assert.AreEqual(220, orders[1].SumAmount);

換個方式查詢，只會取出一筆資料

   IList<[creditcardorder]> orders = session.CreateCriteria<[creditcardorder]>().List<[creditcardorder]>();

   foreach (Order order in orders)
   {
      order.ReCalculate();
   }

   Assert.AreEqual(1, orders.Count);
   Assert.AreEqual(220, orders[0].SumAmount);

查詢語法如下，這似乎讓我們想到有多常寫這種SQL，不如考慮用繼承來解決。

SELECT this_.OrderNo as OrderNo8_0_, this_.ShoppingDate as Shopping3_8_0_, this_.SumAmount as SumAmount8_0_, this_.DiscountAmount as Discount5_8_0_ FROM [Order] this_ WHERE this_.IsCreditCard='Y'