在同一个类中使用OneToMany和ManyToOne
首先想数据库的模型，再想面向对象的模型，然后再来想怎么去做映射，做完映射在想怎么去做CRUD。
1.首先设计数据库的模型
应该是Id、parent_Id.单一父亲，好多孩子这就是一棵树。
数据库首先存在的属性有：
Id<int> parent_Id<int> name<String>
例子：
ID Parent_ID 公司
1 null 分公司1
2 1 分公司2
3 2 分公司2_1
实体类：
package cn.edu.hpu.treeStruct;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
@Entity
public class Org {
private int id;
private String name;
private Set<Org> children=new HashSet<Org>();
private Org parent;
@Id
@GeneratedValue
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;
}
@OneToMany
public Set<Org> getChildren() {
return children;
}
public void setChildren(Set<Org> children) {
this.children = children;
}
@ManyToOne
public Org getParent() {
return parent;
}
public void setParent(Org parent) {
this.parent = parent;
}
}
如何来理解这张表，我们假设这是三张表(都是id、name)。
A、B、C假设B的父亲是A，B的孩子是C，B通过parent_id去找父亲A，
C通过parent_id去找父亲B。只不过是多张表合成一张表。然后每一个人
的parent_id都去参考另外一条记录了，也就是参考自身这张表。是
一对多和多对一的双向关联关系。
测试，输出的建表sql语句：
alter table Org
drop
foreign key FK136C4424F8DF5
drop table if exists Org
create table Org (
id integer not null auto_increment,
name varchar(255),
parent_id integer,
primary key (id)
)
alter table Org
add index FK136C4424F8DF5 (parent_id),
add constraint FK136C4424F8DF5
foreign key (parent_id)
references Org (id)
添加测试：
@Test
public void testTreeAdd(){
Org o=new Org();
o.setName("总公司");
Org o1=new Org();
o1.setName("分公司1");
Org o2=new Org();
o2.setName("分公司2");
Org o11=new Org();
o11.setName("分公司1下公司1");
Org o12=new Org();
o12.setName("分公司1下公司2");
o.getChildren().add(o1);
o.getChildren().add(o2);
o.getChildren().add(o11);
o.getChildren().add(o12);
o11.setParent(o1);
o12.setParent(o1);
o1.setParent(o);
o2.setParent(o);
//可以先存父亲，再存孩子，也可以让Org的Childern设置CascadeType.ALL
//这里用的是设置CascadeType.ALL，所以只存父亲就可以了
sessionFactory=new AnnotationConfiguration().configure().buildSessionFactory();
Session session=sessionFactory.openSession();
session.beginTransaction();
session.save(o);
session.getTransaction().commit();
session.close();
}
输出的sql语句：
Hibernate:
insert
into
Org
(name, parent_id)
values
(?, ?)
Hibernate:
insert
into
Org
(name, parent_id)
values
(?, ?)
Hibernate:
insert
into
Org
(name, parent_id)
values
(?, ?)
Hibernate:
insert
into
Org
(name, parent_id)
values
(?, ?)
Hibernate:
insert
into
Org
(name, parent_id)
values
(?, ?)
Hibernate:
update
Org
set
name=?,
parent_id=?
where
id=?
Hibernate:
update
Org
set
name=?,
parent_id=?
where
id=?
结果：
id name parent_id
1 总公司 null
2 分公司1下公司1 5
3 分公司1下公司2 5
4 分公司2 1
5 分公司1 1
读取测试(用递归)：
简单的方式就是Org的Childern设置fetch=FetchType.EAGER，最好的方式是递归。
首先用fetch=FetchType.EAGER测试：
@Test
public void testTreeLoad(){
sessionFactory=new AnnotationConfiguration().configure().buildSessionFactory();
Session session=sessionFactory.openSession();
session.beginTransaction();
Org o =(Org) session.load(Org.class,1);
Treeprint(o);
session.getTransaction().commit();
session.close();
}
private void Treeprint(Org o) {
System.out.println(o.getName());
for(Org child:o.getChildren()){
Treeprint(child);
}
}
测试结果：
Hibernate:
select
org0_.id as id0_1_,
org0_.name as name0_1_,
org0_.parent_id as parent3_0_1_,
children1_.parent_id as parent3_0_3_,
children1_.id as id3_,
children1_.id as id0_0_,
children1_.name as name0_0_,
children1_.parent_id as parent3_0_0_
from
Org org0_
left outer join
Org children1_
on org0_.id=children1_.parent_id
where
org0_.id=?
Hibernate:
select
children0_.parent_id as parent3_0_1_,
children0_.id as id1_,
children0_.id as id0_0_,
children0_.name as name0_0_,
children0_.parent_id as parent3_0_0_
from
Org children0_
where
children0_.parent_id=?
Hibernate:
select
children0_.parent_id as parent3_0_1_,
children0_.id as id1_,
children0_.id as id0_0_,
children0_.name as name0_0_,
children0_.parent_id as parent3_0_0_
from
Org children0_
where
children0_.parent_id=?
Hibernate:
select
children0_.parent_id as parent3_0_1_,
children0_.id as id1_,
children0_.id as id0_0_,
children0_.name as name0_0_,
children0_.parent_id as parent3_0_0_
from
Org children0_
where
children0_.parent_id=?
Hibernate:
select
children0_.parent_id as parent3_0_1_,
children0_.id as id1_,
children0_.id as id0_0_,
children0_.name as name0_0_,
children0_.parent_id as parent3_0_0_
from
Org children0_
where
children0_.parent_id=?
总公司
分公司1
分公司1下公司2
分公司1下公司1
分公司2
如何展现成树状？
@Test
public void testTreeLoad(){
sessionFactory=new AnnotationConfiguration().configure().buildSessionFactory();
Session session=sessionFactory.openSession();
session.beginTransaction();
Org o =(Org) session.load(Org.class,1);
Treeprint(o,0);
session.getTransaction().commit();
session.close();
}
private void Treeprint(Org o,int level) {
//level是构建树状前的缩进
String preStr="";
for(int i=0;i<level;i++){
preStr+="----";
}
System.out.println(preStr+o.getName());
for(Org child:o.getChildren()){
Treeprint(child,level+1);
}
}测试结果：总公司
----分公司2
----分公司1
--------分公司1下公司2
--------分公司1下公司1
如果这棵树非常大我们用EAGER就不合适了，需要把它去掉了，什么时候需要(get)才从数据库拿出。
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