在本章中，我们将讨论映射组件.在NHibernate中，组件是一个值对象.它没有自己的身份.
这个例子就是钱币，钱包或者钱币钱包可能有钱，但这笔钱的确切身份是无关紧要的.
它没有自己的主键，但组件本身是持久的与拥有对象在同一个表中.
让我们看一个学生有一个地址的简单例子，与位置等级相关联的对象.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace NHibernateDemoApp {
class Student {
public virtual int ID { get; set; }
public virtual string LastName { get; set; }
public virtual string FirstName { get; set; }
public virtual StudentAcademicStanding AcademicStanding { get; set; }
public virtual Location Address { get; set; }
}
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 StudentAcademicStanding {
Excellent,
Good,
Fair,
Poor,
Terrible
}
}现在，我们还需要通过执行以下查询来更新数据库，该查询首先删除Student表，然后创建一个新表，它还将包含Location类的列.
DROP TABLE [dbo].[Student] CREATE TABLE [dbo].[Student] ( [ID] INT IDENTITY (1, 1) NOT NULL, [LastName] NVARCHAR (MAX) NULL, [FirstMidName] NVARCHAR (MAX) NULL, [AcademicStanding] NCHAR(10) NULL, [Street] NVARCHAR (100) NULL, [City] NVARCHAR (100) NULL, [Province] NVARCHAR (100) NULL, [Country] NVARCHAR (100) NULL, CONSTRAINT [PK_dbo.Student] PRIMARY KEY CLUSTERED ([ID] ASC) );
现在映射那些不直接属于Student类的列，但它们是Location类的属性，Location类对象是在student类中定义的.我们需要一个组件来正确映射它.让我们在 student.hbm.xml 文件中创建一个组件，如下面的代码所示.
<?xml version = "1.0" encoding = "utf-8" ?> <hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" assembly = "NHibernateDemoApp" namespace = "NHibernateDemoApp"> <class name = "Student"> <id name = "ID"> <generator class = "native"/> </id> <property name = "LastName"/> <property name = "FirstName" column = "FirstMidName" type = "String"/> <property name = "AcademicStanding"/> <component name = "Address"> <property name = "Street"/> <property name = "City"/> <property name = "Province"/> <property name = "Country"/> </component> </class> </hibernate-mapping>
此组件是地址，它具有这些不同的属性.有了这些信息，NHibernate现在已足够实际映射它了.
现在这里是Program.cs文件，其中创建并初始化一个新的学生对象，然后保存到数据库.然后它将从数据库中检索列表.
using HibernatingRhinos.Profiler.Appender.NHibernate;
using NHibernate.Cache;
using NHibernate.Caches.SysCache;
using NHibernate.Cfg;
using NHibernate.Dialect;
using NHibernate.Driver;
using NHibernate.Linq;
using System;
using System.Linq;
using System.Reflection;
namespace NHibernateDemoApp {
class Program {
static void Main(string[] args) {
NHibernateProfiler.Initialize();
var cfg = new Configuration();
String Data Source = asia13797\\sqlexpress;
String Initial Catalog = NHibernateDemoDB;
String Integrated Security = True;
String Connect Timeout = 15;
String Encrypt = False;
String TrustServerCertificate = False;
String ApplicationIntent = ReadWrite;
String MultiSubnetFailover = False;
cfg.DataBaseIntegration(x = > { x.ConnectionString = "Data Source +
Initial Catalog + Integrated Security + Connect Timeout + Encrypt +
TrustServerCertificate + ApplicationIntent + MultiSubnetFailover";
x.Driver<SqlClientDriver>();
x.Dialect<MsSql2008Dialect>();
});
cfg.AddAssembly(Assembly.GetExecutingAssembly());
var sefact = cfg.BuildSessionFactory();
using (var session = sefact.OpenSession()) {
using (var tx = session.BeginTransaction()) {
var student1 = new Student {
ID = 1,
FirstName = "Allan",
LastName = "Bommer",
AcademicStanding = StudentAcademicStanding.Poor,
Address = new Location {
Street = "123 Street",
City = "Lahore",
Province = "Punjab",
Country = "Pakistan"
}
};
session.Save(student1);
tx.Commit();
var students = session.Query<Student>().ToList<Student>();
Console.WriteLine("\nFetch the complete list again\n");
foreach (var student in students) {
Console.WriteLine("{0} \t{1} \t{2} \t{3} \t{4} \t{5} \t{6} \t{7}",
student.ID,
student.FirstName,
student.LastName,
student.AcademicStanding,
student.Address.Street,
student.Address.City,
student.Address.Province,
student.Address.Country
);
}
}
Console.ReadLine();
}
}
}
}现在我们可以运行这个应用程序，NHibernate可以保存这些值到数据库.当您运行该应用程序时，您将看到以下输出.
Fetch the complete list again 2 Allan Bommer Poor 123 Street Lahore Punjab Pakistan
以下是数据库中的值.
这些组件允许我们将数据库表中的列分离到它们各自的类中.
这里要注意的另一件事是因为Location是一个类，它不是一个实体.
它是一个值类型对象，它没有自己的主键.
它与...保存在同一个表中包含它的学生.
这就是我们在这里使用该组件的原因.
这样可以灵活地更改我们的类层，如何定义类以及如何布局数据库.
