Hibernate关联关系映射-----单向多对一映射配置

   在这里举了一个不太恰当的例子：双亲和孩子。当然举这个例子也有一定的道理，一个孩子至少有两个parent，但是例子只是例子，重点不是例子而是配置方法。下面我们看一下配置的详细步骤：

实体：

package uni.many2one;

public class Child {
	private int id;
	private String name;

	public Child(int id, String name) {
		super();
		this.id = id;
		this.name = name;
	}

	public Child() {
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
}

package uni.many2one;


public class Parent {
	private int id;
	private String name;
	private Child child;

	public Parent() {
	}

	public Parent(int id, String name, Child child) {
		super();
		this.id = id;
		this.name = name;
		this.child = child;
	}

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Child getChild() {
		return child;
	}

	public void setChild(Child child) {
		this.child = child;
	}

}

　　

hbm配置文件：

<hibernate-mapping>
	<class name="uni.many2one.Child">
		<id name="id" column="did">
			<generator class="native" />
		</id>
		<property name="name" type="string" column="name"></property>
	</class>

</hibernate-mapping>
<hibernate-mapping>
	<!-- <class name="org.hibernate.wk.Student" dynamic-insert="true" dynamic-update="true"> -->
	<class name="uni.many2one.Parent">
		<id name="id" column="did">
			<generator class="native" />
		</id>
		<property name="name" type="string" column="name"></property>
		
		<!-- configure the many to one association -->
		<many-to-one name="child" column="child"></many-to-one>
	</class>

</hibernate-mapping>

　　

测试文件：

public void testAdd() {
		SessionFactory sf = HibernateUtil.getSessionFactory();
		Session session = sf.getCurrentSession();
		session.beginTransaction();
		Child c1 = new Child();
		c1.setName("child1");

		Parent mother = new Parent();
		mother.setName("Mother");
		mother.setChild(c1);
		
		Parent father = new Parent();
		father.setName("Daddy");
		father.setChild(c1);
		
		session.save(c1);
		session.save(mother);
		session.save(father);
		session.beginTransaction().commit();
		
	}

　　

测试结果：

Hibernate: insert into Child (name) values (?)
Hibernate: insert into Parent (name, child) values (?, ?)
Hibernate: insert into Parent (name, child) values (?, ?)

　　

这里我没有贴出来表的结构，因为表hibernate会自动生成的，多以就没有浪费地方。

hibernate关联关系映射的配置中务必要搞清楚谁是主动配置方，谁是被配置的，如果搞不清楚很容易在实体操作的时候出现错误。尤其是在保存实体的时候，很容易出现引用为空的情况。