《NHibernate 3.0Cookbook》：70实用教程解锁完整生态

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"《Packtpub.NHibernate.3.0.Cookbook.Oct.2010.pdf》是一本深入浅出的NHibernate 3.0技术指南，由Jason Dentler编著，于2010年10月出版。这本书汇集了70个实用且强大的NHibernate解决方案，涵盖了NHibernate生态系统中的全面知识，旨在帮助读者在实际项目中充分利用这个强大的ORM框架。该书的特点在于其社区经验和实践提炼，旨在为开发者提供实战技巧和最佳实践，帮助他们解决在使用NHibernate 3.0过程中可能遇到的各种复杂问题。它不仅介绍了基础概念，还包括了高级特性如查询优化、事务管理、多态映射、延迟加载等深度内容。通过阅读这本书，读者可以了解到如何设计高效的数据访问策略，以及如何处理常见性能瓶颈和并发控制。版权方面，Packt Publishing明确声明所有权利保留，未经书面许可，不得以任何形式复制、存储或传输书中的内容。尽管作者和出版社尽力确保信息的准确性，但本书不保证完全无误，且不承担因使用书中的信息导致的直接或间接损失的责任。此外，尽管书中尝试正确地标记商标信息，但Packt Publishing不能对此保证准确性，因为商标信息可能会随着时间变化而有所调整。这本书是针对初学者和有经验的NHibernate开发者的一份实用资源，适合那些希望提升对NHibernate 3.0理解和应用能力的专业人士参考。通过阅读这本书，读者将获得一个系统性的学习路径，以便更好地融入到现代企业级应用开发中。"

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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