NHibernate - 延遲載入

本章將介紹延遲載入功能。它是一個完全不同的概念，預設情況下NHibernate不啟用延遲載入，例如，如果您載入一個客戶，它不會載入所有訂單。

訂單集合將在需要時載入。
任何關聯，無論是多對一還是集合，預設情況下都是延遲載入的，它需要一個**開啟的ISession**。
如果您已關閉會話，或者已提交事務，您可能會收到一個延遲載入異常，因為它無法載入這些附加物件。
您必須注意延遲載入以及您實際需要多少資料。
您可以為整個關聯關閉延遲載入，或者您可以將lazy設定為false，或者您可以指定一個獲取策略。

這是**Program.cs**檔案的實現。

using System; 
using System.Data; 
using System.Linq; 
using System.Reflection; 

using HibernatingRhinos.Profiler.Appender.NHibernate; 
using NHibernate.Cfg; 
using NHibernate.Dialect; 
using NHibernate.Driver; 
using NHibernate.Linq;

namespace NHibernateDemo { 

   internal class Program { 
	
      private static void Main() { 
		
         var cfg = ConfigureNHibernate(); 
         var sessionFactory = cfg.BuildSessionFactory();
         
         Guid id; 
         using(var session = sessionFactory.OpenSession()) 
			
         using(var tx = session.BeginTransaction()) {
            var newCustomer = CreateCustomer(); 
            Console.WriteLine("New Customer:"); 
            Console.WriteLine(newCustomer); 
            session.Save(newCustomer); 
            id = newCustomer.Id; 
            tx.Commit(); 
         }
         
         using(var session = sessionFactory.OpenSession()) 
			
         using(var tx = session.BeginTransaction()) { 
            var reloaded = session.Load<Customer>(id); 
            Console.WriteLine("Reloaded:"); 
            Console.WriteLine(reloaded); 
            Console.WriteLine("The orders were ordered by: "); 
            
            foreach (var order in reloaded.Orders) { 
               Console.WriteLine(order.Customer); 
            } 
				
            tx.Commit(); 
         }
			
         Console.WriteLine("Press <ENTER> to exit..."); 
         Console.ReadLine(); 
      }
		
      private static Customer CreateCustomer() { 
         
         var customer = new Customer { 
            FirstName = "John", 
            LastName = "Doe", 
            Points =100, 
            HasGoldStatus = true, 
            MemberSince = new DateTime(2012, 1, 1),
            CreditRating = CustomerCreditRating.Good,
            AverageRating = 42.42424242, 
            Address = CreateLocation() 
         }; 
			
         var order1 = new Order { Ordered = DateTime.Now }; 
         customer.AddOrder(order1); 
         
         var order2 = new Order { 
            Ordered = DateTime.Now.AddDays(-1), 
            Shipped = DateTime.Now, 
            ShipTo = CreateLocation() 
         }; 
			
         customer.AddOrder(order2); return customer; 
      }
		
      private static Location CreateLocation() { 
         return new Location { 
            Street = "123 Somewhere Avenue", 
            City = "Nowhere", 
            Province = "Alberta", 
            Country = "Canada" 
         }; 
      }
		
      private static Configuration ConfigureNHibernate() { 
		
         NHibernateProfiler.Initialize(); 
         var cfg = new Configuration(); 
         
         cfg.DataBaseIntegration(x => { 
            x.ConnectionStringName = "default"; 
            x.Driver<SqlClientDriver>(); 
            x.Dialect<MsSql2008Dialect<(); 
            x.IsolationLevel = IsolationLevel.RepeatableRead; 
            x.Timeout = 10;
            x.BatchSize = 10;
         }); 
         
         cfg.SessionFactory().GenerateStatistics();
         cfg.AddAssembly(Assembly.GetExecutingAssembly()); 
         return cfg; 
      } 
   } 
}

為了理解這一點，讓我們執行應用程式並檢視NHibernate Profiler。

您可以看到，我們有Select From Customer，給定特定的客戶ID，然後我們在實際訪問該客戶的集合時還有另一個Select From Orders表。

所以我們對資料庫進行了兩次往返。現在，有時我們想最佳化它。為此，讓我們轉到**customer.hbm.xml**檔案並新增一個獲取策略，並要求它執行join fetch。

<?xml version = "1.0" encoding = "utf-8" ?> 
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" assembly = "NHibernateDemo"
   namespace = "NHibernateDemo"> 
   
   <class name = "Customer"> 
	
      <id name = "Id"> 
         <generator class = "guid.comb"/> 
      </id> 
   
      <property name = "FirstName"/> 
      <property name = "LastName"/> 
      <property name = "AverageRating"/> 
      <property name = "Points"/> 
      <property name = "HasGoldStatus"/> 
      <property name = "MemberSince" type = "UtcDateTime"/> 
      <property name = "CreditRating" type = "CustomerCreditRatingType"/>
      
      <component name = "Address"> 
         <property name = "Street"/> 
         <property name = "City"/> 
         <property name = "Province"/> 
         <property name = "Country"/> 
      </component>
      
      <set name = "Orders" table = "`Order`" cascade = "all-delete-orphan" 
         fetch = "join"> 
         <key column = "CustomerId"/> 
         <one-to-many class = "Order"/> 
      </set> 
   
   </class> 
</hibernate-mapping>

您可以看到我們沒有更改應用程式中的任何程式碼，我們只是在**customer.hbm.xml**中添加了一個獲取策略。讓我們再次執行此應用程式，它的行為仍然完全相同。讓我們看看NHibernate Profiler。

之前，程式對資料庫進行了兩次往返，現在只有一次，這是因為它在這裡執行了一個左外部連線。
我們可以看到它根據客戶ID在客戶表和訂單表之間執行了一個左外部連線，因此它能夠一次載入所有這些資訊。
我們節省了一次資料庫往返。
缺點是客戶資訊會在兩行上重複，這就是SQL左外部連線的工作方式。
因此，使用獲取策略，我們提取了更多資料，並節省了一次往返。

您也可以在查詢級別執行此操作，因此讓我們轉到**Program.cs**檔案並檢視簡化過載的示例。

using(var session = sessionFactory.OpenSession()) 

using(var tx = session.BeginTransaction()) { 
   //var query = from customer in session.Query<Customer>() 
   // select customer; 
   //var reloaded = query.Fetch(x => x.Orders).ToList();
	
   var reloaded = session.Load<Customer>(id); 
   Console.WriteLine("Reloaded:"); 
   Console.WriteLine(reloaded); 
   Console.WriteLine("The orders were ordered by: "); 
   
   foreach (var order in reloaded.Orders) { 
      Console.WriteLine(order.Customer); 
   } 
	
   tx.Commit(); 
}

在這裡，我們正在透過customer進行載入。現在讓我們將其更改為查詢，我們將使用如下程式碼所示的linq查詢。

using(var session = sessionFactory.OpenSession()) 

using(var tx = session.BeginTransaction()) {
   var query = from customer in session.Query<Customer>() 
   where customer.Id == id select customer; 
   var reloaded = query.Fetch(x => x.Orders).ToList().First();
	
   Console.WriteLine("Reloaded:"); 
   Console.WriteLine(reloaded); 
	
   tx.Commit();
}

讓我們也從**customer.hbm.xml**檔案中刪除獲取策略。

<?xml version = "1.0" encoding = "utf-8" ?> 
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" assembly = "NHibernateDemo"
   namespace = "NHibernateDemo"> 

   <class name = "Customer"> 
	
      <id name = "Id"> 
         <generator class = "guid.comb"/> 
      </id> 
   
      <property name = "FirstName"/> 
      <property name = "LastName"/>
      <property name = "AverageRating"/> 
      <property name = "Points"/> 
      <property name = "HasGoldStatus"/> 
      <property name = "MemberSince" type = "UtcDateTime"/> 
      <property name = "CreditRating" type = "CustomerCreditRatingType"/>
   
      <component name = "Address"> 
         <property name = "Street"/> 
         <property name = "City"/> 
         <property name = "Province"/> 
         <property name = "Country"/> 
      </component>
   
      <set name = "Orders" table = "`Order`" cascade = "all-delete-orphan"> 
         <key column = "CustomerId"/> 
         <one-to-many class = "Order"/> 
      </set> 
   
   </class> 
</hibernate-mapping>

讓我們再次執行此應用程式，您將看到以下輸出。

New Customer:
John Doe (00000000-0000-0000-0000-000000000000)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Unspecified)
   CreditRating: Good
   AverageRating: 42.42424242

   Orders:
      Order Id: 00000000-0000-0000-0000-000000000000
      Order Id: 00000000-0000-0000-0000-000000000000

Reloaded:
John Doe (6ebacd17-f9ba-4ad8-9817-a5bb01112a5a)
   Points: 100
   HasGoldStatus: True
   MemberSince: 1/1/2012 12:00:00 AM (Utc)
   CreditRating: Good
   AverageRating: 42.4242

   Orders:
      Order Id: 16a6596b-d56e-41c7-9681-a5bb01112a60
      Order Id: d41d615b-0f21-4032-81db-a5bb01112a61
		
Press <ENTER> to exit...

現在讓我們看看NHibernate Profiler，您可以看到我們再次進行了這個渴望的join fetch，但是這次是基於查詢的。

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