NHibernate – 延迟加载
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，并且 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。
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订单集合将按需加载。
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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，它需要一个Open ISession。
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如果您已经关闭了会话，或者您已经提交了您的事务，您可能会得到一个延迟加载异常，它无法拉入这些额外的对象。
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您必须小心延迟加载以及您实际需要的数据量。
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您可以关闭整个关联的延迟加载，或者您可以设置延迟等于 false，或者您也可以指定一个获取策略。
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;
}
}
}
为了理解这一点，让我们运行应用程序并查看 NHibernate Profiler。
正如您所看到的，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。
所以我们有 2 次到数据库的往返。现在，有时，我们想要优化它。为此，让我们转到customer.hbm.xml文件并添加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。让我们再次运行这个应用程序，它的行为仍然完全相同。让我们看看 NHibernate Profiler。
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以前，程序有两次到数据库的往返，现在，它只有一次，那是因为它在这里进行了左外连接。
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我们可以看到它正在根据客户 ID 在客户表和订单表之间进行左外连接，因此，它能够一次加载所有这些信息。
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我们已经保存了 1 次到数据库的往返。
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不利的一面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。
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因此，通过获取策略，我们拉回了更多数据并节省了往返。
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();
}
在这里，我们正在做客户的负载。现在让我们将其更改为查询，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();
}
让我们也从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...
现在让我们看看 NHibernate Profiler，您可以看到我们再次发生了这种 Eager join fetch，但这一次，它是基于查询的。
