NHibernate 3.0实战指南：Cookbook解析

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"Packtpub.NHibernate.3.0.Cookbook.Oct.2010" 本书《Packtpub.NHibernate.3.0.Cookbook.Oct.2010》是针对NHibernate 3.0框架的一本实战指南，作者Jason Dentler精心编撰了70个强大的解决方案，涵盖了NHibernate生态系统中的各种应用场景。这本书旨在帮助开发者深入理解和有效地运用NHibernate，提升他们在实际项目中的开发效率。 NHibernate是.NET平台上的一款流行的对象关系映射（ORM）框架，它允许开发者用面向对象的方式来处理数据库操作，减少了直接编写SQL语句的工作量。3.0版本带来了许多改进和新特性，包括性能优化、更丰富的API以及对.NET Framework 4.0的支持。书中的70个配方（recipes）涵盖以下关键知识点： 1. **安装与配置**：如何设置NHibernate项目，包括依赖库的添加、配置文件的编写，以及数据库连接的管理。 2. **对象关系映射（ORM）基础**：介绍实体类的设计，属性映射，以及如何通过XML或Fluent NHibernate进行映射配置。 3. **会话管理**：理解会话的概念，学习如何打开、关闭、提交和回滚事务，以及处理并发控制和事务隔离级别。 4. **查询**：讲解HQL（NHibernate查询语言）、 Criteria API以及Linq to NHibernate，以及如何进行分页、排序和过滤数据。 5. **关联映射**：一对一、一对多、多对一、多对多关系的建立和管理，包括级联操作和懒加载策略。 6. **性能优化**：缓存机制的使用，如第一级缓存和第二级缓存，以及查询优化技巧，减少数据库访问次数。 7. **复杂查询与子查询**：如何处理复杂的业务逻辑，使用子查询和联合查询来获取所需数据。 8. **事件和拦截器**：利用NHibernate的事件系统和拦截器进行扩展，实现日志记录、权限控制等自定义行为。 9. **持久化策略**：了解不同的持久化策略，如脏检查、版本控制和乐观锁，以及如何处理实体状态。 10. **映射高级特性**：枚举类型映射、自定义类型、集合类型的高级用法，以及如何映射数据库特定的功能，如存储过程。 11. **映射关系数据库模式**：如何将现有数据库模式映射到实体模型，以及如何自动生成数据库结构。 12. **测试与调试**：使用NHibernate进行单元测试的策略，以及在开发过程中如何有效地调试NHibernate。通过阅读这本书，开发者可以掌握NHibernate的核心功能，并学会如何在实际项目中灵活应用。无论是初学者还是有经验的开发人员，都能从中受益，提升自己的ORM技能。

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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