NHibernate 3.0深度探索：权威指南

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"NHibernate 3.0 - 一本关于NHibernate 3.0的PDF学习资源，由Jason Dentler撰写，包含70个强大的解决方案，全面涵盖了NHibernate生态系统的各个方面。" NHibernate 3.0是.NET框架下的一款流行的开源对象关系映射（ORM）工具，它允许开发人员在SQL数据库和面向对象的编程之间建立桥梁，从而简化数据访问层的开发。此资源，"NHibernate 3.0 Cookbook"，提供了70个强大且实用的食谱或示例，旨在帮助开发者更有效地利用NHibernate的各种功能。该书可能涵盖以下关键知识点： 1. **安装与配置**：如何在项目中安装NHibernate，设置配置文件，以及连接数据库。 2. **实体映射**：学习如何使用XML或 Fluent NHibernate 定义对象到数据库表的映射，包括属性映射、关系映射（一对一、一对多、多对多）、继承映射等。 3. **会话管理**：理解NHibernate的会话概念，包括何时打开和关闭会话，以及如何在事务中使用会话。 4. **查询API**：HQL（Hibernate Query Language）的基础和高级用法，以及Criteria API的使用，用于执行数据库查询。 5. **CRUD操作**：创建（Create）、读取（Read）、更新（Update）和删除（Delete）对象的基本方法，以及批处理操作。 6. **性能优化**：缓存策略（一级缓存和二级缓存），延迟加载（Lazy Loading）和批处理更新，以提高应用程序性能。 7. **复杂查询和关联**：使用JOINs，子查询，分页，排序，以及处理复杂关联结构。 8. **事件和拦截器**：通过NHibernate事件系统来监听和处理对象生命周期事件，或者使用拦截器实现自定义行为。 9. **持久化策略**：了解如何处理乐观锁和悲观锁，以及时间戳和版本号的使用。 10. **映射第三方库和复杂类型**：如何将NHibernate与其他库（如DateTimeOffset，自定义类型）集成。 11. **错误排查和最佳实践**：解决常见的NHibernate问题，以及遵循的开发和设计模式。此资源作为一本Cookbook，会提供详细的步骤和代码示例，帮助读者逐步掌握NHibernate 3.0的用法，并在实际项目中应用这些技巧。尽管这本书是英文版，但内容清晰且实用，对于有志于深入学习NHibernate的.NET开发者来说，是一份宝贵的参考资料。

Models and Mappings

In this chapter, we will cover the following topics:

f Mapping a class with XML

f Creating class hierarchy mappings

f Mapping a one-to-many relationship

f Setting up a base entity class

f Bidirectional one-to-many class relationships

f Handling versioning and concurrency

f Creating mappings uently

f Mapping with ConfORM

Introduction

NHibernate is a popular, mature, open source object / relational mapper (ORM) based on

Java's Hibernate project. ORMs, such as LINQ to SQL, Entity Framework, and NHibernate,

translate between the database's relational model of tables, columns, and keys to the

application's object model of classes and properties.

The NHibernate homepage,

http://NHForge.org, contains blog posts, a wiki, the

complete reference documentation, and a bug tracker. Support is available through the

very active nhusers Google group at http://groups.google.com/group/nhusers.

The NHibernate source code is hosted on SourceForge at http://sourceforge.net/

projects/nhibernate/

. Precompiled binaries of NHibernate releases are also available

on SourceForge.

Models and Mappings

7. In the editor, enter the following XML in Product.hbm.xml. Let the IntelliSense

guide you.

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"

assembly="Eg.Core"

namespace="Eg.Core">

</id>

<property name="UnitPrice" not-null="true"

type="Currency" />

</class>

</hibernate-mapping>

How it works...

In this recipe, we begin by creating our model. The model is the collection of classes that will

be persisted or stored in the database. A persistent class is any class that will be persisted.

An entity class is a persistent class with an ID. An instance of an entity class is called an

entity. So far, our model only contains the Product entity class. We will expand on this model

over the next few recipes.

Notice that our Product class looks just like any other Plain Old CLR Object (POCO) class.

One of the strongly held design decisions in NHibernate is that all entity classes should be

persistence ignorant, that is, they should not know about, or be dependent on NHibernate.

Let's examine the Id property a little closer. The Id property of each Product instance will

contain the primary key value from the database. In NHibernate, this is named the persistent

object identier (POID). Just as the primary key value uniquely identies a row in a database

table, the POID will uniquely identify an entity in memory.

If you are new to NHibernate, this protected setter may look strange to you.

public virtual Guid Id { get; protected set; }

This is a shorthand way to limit access to the Id property. Code outside of the Product class

is unable to change the value of the Id property. However, NHibernate sets properties using

highly optimized reection, ignoring the protected restriction. This keeps your application

from inadvertently altering this value.

Chapter 1

Next, we create our mapping for the Product entity class. Visual Studio uses the

nhibernate-mapping.xsd schema to provide IntelliSense while completing this mapping.

As a general rule, all NHibernate mapping les end with a.hbm.xml extension, and have a

build action of Embedded Resource. NHibernate searches through the embedded resources

in your assembly, loading each one with this extension.

One of the most common mistakes in mapping is forgetting to set the build

action to Embedded Resource. This leads to the "No Persister for class"

MappingException.

Let's break down this XML mapping. Every XML mapping document contains a single

hibernate-mapping element. The xmlns attribute sets the XML namespace. Along

with the schema in our Schema folder, Visual Studio uses this to enable IntelliSense

inside NHibernate mappings.

The

assembly attribute tells NHibernate which assembly, by default, contains our types.

Similarly, the namespace attribute sets the default .NET namespace types in this mapping le.

Together, they allow us to use the simple name Product instead of the full assembly qualied

name of Eg.Core.Product, Eg.Core. Inside the hibernate-mapping element, we have

a class element. The name attribute tells NHibernate that this class element denes the

mapping for our entity class Product.

The

Id element denes the POID. The name attribute refers to the Id property of our

Product class. It is case-sensitive, just as in the C# language.

The

generator element denes how NHibernate will generate POIDs. In this case, we've told

NHibernate to use the guid.comb algorithm. Several other options exist.

The

property elements dene properties on our Product class. Each name attribute

matches the name of a property on our Product class. By default, NHibernate allows null

values. Adding not-null="true" tells NHibernate to disallow null values.

Avoid redundant mappings

In general, it's best to keep your mappings as short and concise as possible.

NHibernate intelligently scans your model and combines this knowledge

with the information provided in the mapping. In most cases, specifying

the types of properties in your mappings only creates redundancies that

must be maintained. The default table name matches the class name,

and each column name matches the corresponding property by default.

It's not necessary to specify this information again. Similarly, you should

avoid setting an attribute in your mapping when it matches an NHibernate

default. For example, adding not-null="false" to each of your

properties is redundant, and makes your mapping difcult to read.

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