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，无论是多对一关联或者，默认情况下，集合是延迟加载的，它需要打开ISession .
如果您已关闭会话，或者您已关闭已经提交了你的事务，你可以得到一个懒惰的加载异常，它无法吸引那些额外的对象.
你必须小心延迟加载和多少您实际需要的数据.
您可以关闭整个关联的延迟加载，也可以将lazy等于false，或者也可以指定提取策略.
这是 Program.cs 文件的实现.
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;
}
}
}要理解这一点，让我们运行应用程序并查看NHibernate Profiler.
如您所见，我们有Select From客户，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.
所以我们有2次往返数据库.现在，有时，我们想要优化它.为此，我们转到 customer.hbm.xml 文件并添加一个提取策略并要求它进行连接提取.
<?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>
您可以看到我们没有更改应用程序中的任何代码，我们刚刚在 customer.hbm中添加了一个提取策略. XML 的.让我们再次运行这个应用程序，它的行为方式完全相同.让我们来看看NHibernate Profiler.
之前，程序有两次往返数据库，现在，它只有一个，那是因为它在这里进行左外连接.
我们可以看到它在客户表和订单表之间根据客户ID进行左外连接，因此，它可以一次加载所有这些信息.
我们已经将1次往返保存到数据库中.
缺点是客户信息将会在两行上都是重复的，这就是SQL左外连接的工作方式.
因此，通过提取策略，我们将撤回更多的数据和我们正在保存往返.
您也可以在查询级别执行此操作，所以让我们转到 Program.cs 文件并查看更简单的重新加载例如.
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();
}在这里，我们正在由客户负责.现在让我们将其更改为一个查询，我们将使用链接查询，如下面的代码所示.
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();
}我们还要从 customer.hbm.xml 文件中删除提取策略.
<?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>
让我们再次运行此应用程序，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...
现在让我们看一下NHibernate Profiler，你可以看到我们再次发生这种渴望的连接获取，但这一次，它基于查询.
