在本章中，我们将介绍另一个反向关系特征.这是一个有趣的选项，你会在集合上看到它与true相反，它也会让许多开发人员感到困惑.那么让我们谈谈这个选项.要理解这一点，您必须考虑关系模型.假设您使用单个外键进行双向关联.
从关系角度来看，您有一个外键，它代表客户订购和订单给客户.
从OO模型中，您可以使用这些参考进行单向关联.
没有任何内容表明两个单向关联在数据库中表示相同的双向关联.
问题这里是NHibernate没有足够的信息知道 customer.orders 和 order.customer 代表数据库中的相同关系.
我们需要提供 inverse equals true 作为提示，这是因为单向关联使用相同的数据.
如果我们尝试保存这些有2个引用的关系，NHibernate会尝试两次更新该引用.
反向等于true告诉NHibernate忽略关系的哪一方.
当你将它应用到收集方时，NHibernate将始终从另一方更新外键，子对象端.
然后我们只对该外键进行一次更新，我们没有对该数据的其他更新.
这使我们可以防止对外键的这些重复更新，这也有助于我们防止外键违规.
让我们看一下 customer.cs 文件，您将在其中看到 AddOrder 方法，这里的想法是我们现在拥有的这个后退指针从订单返回到客户，需要进行设置.因此，当一个订单被添加到客户时，该客户的后退指针被设置，否则，它将为空，因此我们需要这个以在对象图中将它们正确连接在一起.
using System;
using System.Text;
using Iesi.Collections.Generic;
namespace NHibernateDemo {
public class Customer {
public Customer() {
MemberSince = DateTime.UtcNow; Orders = new HashedSet<Order>();
}
public virtual Guid Id { get; set; }
public virtual string FirstName { get; set; }
public virtual string LastName { get; set; }
public virtual double AverageRating { get; set; }
public virtual int Points { get; set; }
public virtual bool HasGoldStatus { get; set; }
public virtual DateTime MemberSince { get; set; }
public virtual CustomerCreditRating CreditRating { get; set; }
public virtual Location Address { get; set; }
public virtual ISet<Order> Orders { get; set; }
public virtual void AddOrder(Order order) { Orders.Add(order); order.Customer = this; }
public override string ToString() {
var result = new StringBuilder();
result.AppendFormat("{1} {2} ({0})\r\n\tPoints: {3}\r\n\tHasGoldStatus:
{4}\r\n\tMemberSince: {5} ({7})\r\n\tCreditRating: {6}\r\n\tAverageRating:
{8}\r\n", Id, FirstName, LastName, Points, HasGoldStatus, MemberSince,
CreditRating, MemberSince.Kind, AverageRating);
result.AppendLine("\tOrders:");
foreach(var order in Orders) {
result.AppendLine("\t\t" + order);
}
return result.ToString();
}
}
public class Location {
public virtual string Street { get; set; }
public virtual string City { get; set; }
public virtual string Province { get; set; }
public virtual string Country { get; set; }
}
public enum CustomerCreditRating {
Excellent,
VeryVeryGood,
VeryGood,
Good,
Neutral,
Poor,
Terrible
}
}这是 Program.cs 文件实现.
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 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();
}
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;
}
}
}它会将其保存到数据库然后重新加载.现在让我们运行你的应用程序并打开NHibernate Profiler，看看它是如何实际保存它的.
您会注意到我们有3组语句.第一个将插入客户，该客户的ID是Guid，突出显示.第二个语句是插入到订单表中.
您会注意到在那里设置了相同的Customer Id Guid，因此设置了该外键.最后一个语句是更新，它将再次将外键更新为相同的客户ID.
现在的问题是客户有订单，订单有客户，我们没有办法告诉NHibernate它实际上是相同的关系.我们这样做的方法是反等于真.
所以让我们转到我们的 customer.hbm.xml 映射文件并设置反转等于true，如图所示在以下代码中.
<?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" inverse = "true"> <key column = "CustomerId"/> <one-to-many class = "Order"/> </set> </class> </hibernate-mapping>
保存订单时，它会从订单侧设置该外键.现在让我们再次运行这个应用程序并打开NHibernate探查器.
如果我们看看它们是如何插入的，我们在客户中获得插入，并插入订单，但我们没有外键的重复更新，因为它在保存订单时正在更新./p>
现在，您应该注意，如果您只有单向关联，并且它是维持此关系的集合，那么如果反转等于true，则永远不会设置外键，并且这些项永远不会在数据库中设置外键.
如果您查看 Order.hbm.xml 文件中的多对一关系，并且您查找了反向关系，则它实际上没有反向属性.
它总是从子项设置，但如果你有一个多对多的集合，你可以从任何一方设置它.
