Hibernate 一对多关联配置

以Department和Employee为例。

Department类：

public class Department {
	private int id;
	private String name;
	// For one-to-many association
	private Set<Employee> employees;
	public Department() {
	}
	public Department(String name) {
		this.name = name;
	}
	// Getters and setters are omitted

Employee类：

public class Employee {
	private int id;
	private String name;
	public Employee() {
	}
	public Employee(String name) {
		this.name = name;
	}
	// Getters and setters are omitted

Department.hbm.xml:

<hibernate-mapping
	package="com.john.myhibernate.domain">

	<class name="Department">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name" length="20" not-null="true"/>
		<set name="employees" lazy="true" fetch="select">
			<key column="depart_id" not-null="true" foreign-key="slender"></key>
			<one-to-many class="Employee"/>
		</set>
	</class>
</hibernate-mapping>

如果key的column属性不指定，则employee表中没有department的外键。
key的foreign-key属性指定key的名称。
如果one-to-many的class不指定，hibernate生成表时会报错。

Employee.hbm.xml:

<hibernate-mapping package="com.john.myhibernate.domain">
<class name="Employee">
	<id name="id">
		<generator class="native"/>
	</id>
	<property name="name" length="20" not-null="true"/>
</class>
</hibernate-mapping>

1. 测试cascade属性

public void testSave() {
	Session s = null;
	Transaction tx = null;
	
	Department depart = new Department("FDI");
	Employee em1 = new Employee("Jacy");
	Employee em2 = new Employee("Neil");
	Set<Employee> employees = new HashSet<Employee>();
	employees.add(em1);
	employees.add(em2);
	depart.setEmployees(employees);
	
	try {
		s = HibernateUtil.getSession();
		tx = s.beginTransaction();
		s.save(depart);
		tx.commit();
	} catch (HibernateException e) {
		tx.rollback();
		e.printStackTrace();
	} finally {
		if (s != null)
			s.close();
	}
}

结果报错：org.hibernate.TransientObjectException
因为em1和em2是两个瞬时对象。

解决办法是：
a. 在代码里加上s.save(em1); s.save(em2);

	s.save(em1);
	s.save(em2);
	s.save(depart);

或者
b. 给set加上cascade="save-update"或者cascade="all"。

<hibernate-mapping
	<set name="employees" lazy="true" fetch="select" cascade="save-update">
</hibernate-mapping>

2. 测试inverse属性：
set的inverse属性默认是false的，也就是由department维护关系。如果修改为true，执行上面的testSave方法，结果报错：Field 'depart_id' doesn't have a default value
这时关系由Employee维护，但是它没有机会（因为没有调用s.save(em1)和s.save(em2)），那么employee表中的depart_id就会是null值，和not-null=true的约束产生冲突。


3. 测试lazy, fetch属性

public void testQueryLazy() {
	Session s = null;
	
	s = HibernateUtil.getSession();
	Department depart = (Department) s.get(Department.class, 2);
	s.close();
	System.out.println(depart.getName());
	System.out.println(depart.getEmployees());
}

a. 当fetch为select时：
   如果lazy为true，hibernate用SELECT语句查询出Department。当访问Employee时才用SELECT语句查询Employee，如果这时session已经关闭，会导致异常。
   如果lazy为false，hibernate在get方法执行的时候用两个SELECT语句把Department和Employee都查询出来。
b. 当fetch为join时：
   不管lazy的取值，hibernate会进行连表查询，把两个实体都查询出来。