richard_mighty的博客

In this article I want to discuss the lazy loading mechanism provided by NHibernate. It is recommended for maximum flexibility to define all relations in your domain as lazy loadable. This is the default behavior of NHibernate since version 1.2. But this can lead to some undesired effects if querying your data. Let's discuss these effects and how to avoid them. Term Paper AND Research Paper In my previous posts I showed how to prepare your system for NHibernate and how to implement a first NHibernate base application. This post is based on those two articles. Thesis AND Dissertation AND Essay The Domain Let's first define a simple domain. It shows part of an order entry system. I keep this model as simple as possible (a real domain model would be more complex) but it contains all aspects we want to discuss in this post. Below is the class diagram of our model We have an order entity which can be placed by a customer entity. Each order can have many order line entities. Each of the three entity types is uniquely identified by a property Id (surrogate key). The Mapping Files We have to write one mapping file per entity. It is recommended that you always have one mapping per file. Don't forget to set the Build Action of each mapping file to Embedded Resource. People often tend to forget it and the subsequent errors raised by NHibernate are not always obvious. Also do not forget to give the mapping files the correct name, that is *.hbm.xml where * denotes the placeholder for the entity name. The mapping for the Order entity might be implemented as follows <?xml version="1.0" encoding="utf-8" ?> <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="LazyLoadEagerLoad" namespace="LazyLoadEagerLoad.Domain"> <class name="Order" table="Orders"> <id name="Id"> <generator class="guid"/> </id> <property name="OrderNumber"/> <property name="OrderDate"/> <many-to-one name="Customer" /> <set name="OrderLines" cascade="all-delete-orphan" > <key column="OrderId"/> <one-to-many class="OrderLine"/> </set> </class> </hibernate-mapping>Analogous you can implement the mappings for the Customer entity <?xml version="1.0" encoding="utf-8" ?> <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="LazyLoadEagerLoad" namespace="LazyLoadEagerLoad.Domain"> <class name="Customer"> <id name="Id"> <generator class="guid"/> </id> <property name="CompanyName"/> </class> </hibernate-mapping>and finally the mapping for the OrderLine entity. <?xml version="1.0" encoding="utf-8" ?> <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="LazyLoadEagerLoad" namespace="LazyLoadEagerLoad.Domain"> <class name="OrderLine"> <id name="Id"> <generator class="guid"/> </id> <property name="Amount"/> <property name="ProductName"/> </class> </hibernate-mapping>