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【hibernate框架】一对多(多对一)双向CRUD-关于delete2


如果没有关联关系,平常我们使用save、get、update、delete方法即可对实体对象进行增删改查,但是一旦实体对象有关联关系的时候,就不能直接使用save、get、update、delete方法了,就要进行一系列的语句拆分和组合。

牵扯到对一个实体对象做操作,是否要对关联的另外一个实体对象做操作。

这里来探讨一下删除delete“多对一”中“多”这一方的问题:

User.java:

package cn.edu.hpu.one2many;


import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;


@Entity
@Table(name="m_user")
public class User {
private int id;
private String name;
private Group group;

//只要有双向就要指定制定一个属性(mapedby)
//不指定的话会有两个相同的字段产生
@ManyToOne(cascade={CascadeType.ALL},fetch=FetchType.EAGER)
public Group getGroup() {
return group;
}
public void setGroup(Group group) {
this.group = group;
}
@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;
}

}



Group.java:


package cn.edu.hpu.one2many;


import java.util.HashSet;
import java.util.Set;


import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;


@Entity
@Table(name="m_group")
public class Group {
private int id;
private String name;
private Set<User> users=new HashSet<User>();
//选择set的原因是因为,set互相之间不会有重复的
//跟数据库模型比较匹配

@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(mappedBy="group",
cascade={CascadeType.ALL},
fetch=FetchType.LAZY
)
public Set<User> getUsers() {
return users;
}
public void setUsers(Set<User> users) {
this.users = users;
}

}



删除多那一方的测试代码:


@Test
public void testDeleteGroup(){
sessionFactory=new AnnotationConfiguration().configure().buildSessionFactory();
Session s=sessionFactory.openSession();
s.beginTransaction();
Group g=(Group)s.load(Group.class,4);
s.delete(g);
s.getTransaction().commit();
}



输出的sql语句:


说明把与group关联的user全部删除了(u3和u4都是关联g4的,全被删)。原因是因设置了cascade={CascadeType.ALL}。

Hibernate: 

select

group0_.id as id0_0_,

group0_.name as name0_0_

from

m_group group0_

where

group0_.id=?

Hibernate:

select

users0_.group_id as group3_0_1_,

users0_.id as id1_,

users0_.id as id1_0_,

users0_.group_id as group3_1_0_,

users0_.name as name1_0_

from

m_user users0_

where

users0_.group_id=?

Hibernate:

delete

from

m_user

where

id=?

Hibernate:

delete

from

m_user

where

id=?

Hibernate:

delete

from

m_group

where

id=?


如果不想删除关联的u3和u4怎么办呢?




很简单,还是将关联去除(将group的user全部提取出来,去除关联关系,即group设置为null,),然后在删除即可。(但是u3、u4就变成了“垃圾数据”)建议把关联的也删除,根据情况而定了(一般都是根删除,孩子也删除)。



觉得难控制,自己写HQL语句就行了。


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