NHibernate - 延迟加载

在本章中，我们将介绍延迟加载功能。默认情况下，这是一个完全不同的概念，NHibernate 没有延迟加载，例如，如果您加载客户，它不会加载所有订单。

订单集合将按需加载。
任何关联，无论是多对一还是集合，默认情况下都是延迟加载的，它需要 Open ISession。
如果您已关闭会话，或者已提交事务，则可能会出现延迟加载异常，表明无法提取这些其他对象。
您必须小心延迟加载以及您实际需要的数据量。
您可以关闭整个关联的延迟加载，也可以将 lazy equals 设置为 false，或者也可以指定获取策略。

这是 Program.cs文件实施。

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; 
      } 
   } 
}

为了理解这一点，让我们运行应用程序并查看 NHibernate Profiler。

正如您所见，我们有 Select From Customer，给定一个特定的客户 ID，然后我们还有另一个 Select From Orders 表，当它实际访问该客户的集合时。

因此，我们有 2 次往返数据库。现在，有时，我们想要优化这一点。为此，让我们转到 customer.hbm.xml 文件并添加一个获取策略并要求它执行连接获取。

<?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>

如您所见，我们没有更改应用程序中的任何代码，我们只是在 customer.hbm.xml 中添加了一个获取策略。让我们再次运行此应用程序，它仍然表现得完全相同。让我们看看 NHibernate Profiler。

以前，程序需要两次往返数据库，现在只需要一次，这是因为它在这里执行左外连接。
我们可以看到它根据客户 ID 在客户表和订单表之间执行左外连接，因此，它能够一次加载所有信息。
我们节省了一次往返数据库的次数。
缺点是客户信息将在两行上重复，这就是 SQL 左外连接的工作方式。
因此，通过获取策略，我们可以拉回更多数据，并节省往返。

您也可以在查询级别执行此操作，因此让我们转到 Program.cs 文件并查看更简单的重新加载示例。

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(); 
}

这里，我们由客户进行加载。现在让我们将其更改为查询，我们将使用链接查询，如以下代码所示。

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();
}

我们还可以从 customer.hbm.xml 文件中删除获取策略。

<?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>

让我们再次运行该应用程序，您将看到以下输出。

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...

现在让我们看看 NHibernate Profiler，你可以看到我们再次发生了这种急切的连接获取，但这一次，它是基于查询的。

NHibernate 教程

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NHibernate - 延迟加载

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