第一节:数据库表与表之间的关系
1.1 一对多关系
- 1.1.1 什么样的关系属于一对多?
- 一个部门对应多个员工,一个员工只能属于某一个部门
- 一个客户对应多个联系人,一个联系人只能属于某一个客户
- 1.1.2 一对多的建表原则:
在多的一方创建外键指向一的一方的主键。
1.2 多对多关系
- 1.2.1 什么样的关系属于多对多?
- 一个学生可以选择多门课程,一门课程也可以被多个学生选择。
- 一个用户可以选择多个角色,一个角色也可以被多个用户选择。
- 1.2.2 多对多的建表原则:
创建一个中间表,中间表至少有两个字段,分别作为外键指向多对多双方的主键。
1.3 一对一关系(了解)
- 1.3.1 什么样关系属于一对一?
- 一个公司只能有一个注册地址,一个注册地址只能被一个公司注册。
- 1.3.2 一对一建表原则:
- 唯一外键对应
- 主键对应
第二节:Hibernate中一对多表之间映射关系的建立
2.1 创建数据库和表
SQL语句如下:
//客户信息,一
CREATE TABLE `cst_customer` (
`cust_id` bigint(32) NOT NULL AUTO_INCREMENT COMMENT '客户编号(主键)',
`cust_name` varchar(32) NOT NULL COMMENT '客户名称(公司名称)',
`cust_source` varchar(32) DEFAULT NULL COMMENT '客户信息来源',
`cust_industry` varchar(32) DEFAULT NULL COMMENT '客户所属行业',
`cust_level` varchar(32) DEFAULT NULL COMMENT '客户级别',
`cust_phone` varchar(64) DEFAULT NULL COMMENT '固定电话',
`cust_mobile` varchar(16) DEFAULT NULL COMMENT '移动电话',
PRIMARY KEY (`cust_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
//联系人信息:多
CREATE TABLE `cst_linkman` (
`lkm_id` bigint(32) NOT NULL AUTO_INCREMENT COMMENT '联系人编号(主键)',
`lkm_name` varchar(16) DEFAULT NULL COMMENT '联系人姓名',
`lkm_cust_id` bigint(32) DEFAULT NULL COMMENT '客户id',
`lkm_gender` char(1) DEFAULT NULL COMMENT '联系人性别',
`lkm_phone` varchar(16) DEFAULT NULL COMMENT '联系人办公电话',
`lkm_mobile` varchar(16) DEFAULT NULL COMMENT '联系人手机',
`lkm_email` varchar(64) DEFAULT NULL COMMENT '联系人邮箱',
`lkm_qq` varchar(16) DEFAULT NULL COMMENT '联系人qq',
`lkm_position` varchar(16) DEFAULT NULL COMMENT '联系人职位',
`lkm_memo` varchar(512) DEFAULT NULL COMMENT '联系人备注',
PRIMARY KEY (`lkm_id`),
KEY `FK_cst_linkman_lkm_cust_id` (`lkm_cust_id`),
CONSTRAINT `FK_cst_linkman_lkm_cust_id` FOREIGN KEY (`lkm_cust_id`) REFERENCES `cst_customer` (`cust_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
2.2 创建实体类与映射类
- 2.2.1 一的一方的实体类以及映射
Customer实体类
package com.seapp.hibernate.domain;
import java.util.HashSet;
import java.util.Set;
public class Customer {
private Long cust_id;
private String cust_name;
private String cust_source;
private String cust_industry;
private String cust_level;
private String cust_phone;
private String cust_mobile;
//通过ORM方式表示,一个客户对应多个联系人
//放置的多的一方的集合,Hibernate默认使用的是set集合
private Set linkMans = new HashSet<>();
public Customer() {
}
public Long getCust_id() {
return cust_id;
}
public void setCust_id(Long cust_id) {
this.cust_id = cust_id;
}
public String getCust_name() {
return cust_name;
}
public void setCust_name(String cust_name) {
this.cust_name = cust_name;
}
public String getCust_source() {
return cust_source;
}
public void setCust_source(String cust_source) {
this.cust_source = cust_source;
}
public String getCust_industry() {
return cust_industry;
}
public void setCust_industry(String cust_industry) {
this.cust_industry = cust_industry;
}
public String getCust_level() {
return cust_level;
}
public void setCust_level(String cust_level) {
this.cust_level = cust_level;
}
public String getCust_phone() {
return cust_phone;
}
public void setCust_phone(String cust_phone) {
this.cust_phone = cust_phone;
}
public String getCust_mobile() {
return cust_mobile;
}
public void setCust_mobile(String cust_mobile) {
this.cust_mobile = cust_mobile;
}
public Set getLinkMans() {
return linkMans;
}
public void setLinkMans(Set linkMans) {
this.linkMans = linkMans;
}
@Override
public String toString() {
return "Customer{" +
"cust_id=" + cust_id +
", cust_name='" + cust_name + '\'' +
", cust_source='" + cust_source + '\'' +
", cust_industry='" + cust_industry + '\'' +
", cust_level='" + cust_level + '\'' +
", cust_phone='" + cust_phone + '\'' +
", cust_mobile='" + cust_mobile + '\'' +
'}';
}
}
Customer.hbm.xml映射文件
使用set集合来描述Customer.java类中的属性linkMans。在Hibernate的映射文件中,使用
- 2.2.2 多的一方的实体类与映射文件
LinkMan实体类
package com.seapp.hibernate.domain;
/**
* 联系人的实体类
*/
public class LinkMan {
/*
`lkm_id` bigint(32) NOT NULL AUTO_INCREMENT COMMENT '联系人编号(主键)',
`lkm_name` varchar(16) DEFAULT NULL COMMENT '联系人姓名',
`lkm_cust_id` bigint(32) DEFAULT NULL COMMENT '客户id',
`lkm_gender` char(1) DEFAULT NULL COMMENT '联系人性别',
`lkm_phone` varchar(16) DEFAULT NULL COMMENT '联系人办公电话',
`lkm_mobile` varchar(16) DEFAULT NULL COMMENT '联系人手机',
`lkm_email` varchar(64) DEFAULT NULL COMMENT '联系人邮箱',
`lkm_qq` varchar(16) DEFAULT NULL COMMENT '联系人qq',
`lkm_position` varchar(16) DEFAULT NULL COMMENT '联系人职位',
`lkm_memo` varchar(512) DEFAULT NULL COMMENT '联系人备注',
*/
private Long lkm_id;
private String lkm_name;
private Long lkm_cust_id;
private Integer lkm_gender;
private String lkm_phone;
private String lkm_mobile;
private String lkm_email;
private String lkm_qq;
private String lkm_position;
private String lkm_memo;
//通过ORM方式表示,一个联系人只能属于一个客户。
//放置的是一的一方的对象
private Customer customer;
public LinkMan() {
}
public Long getLkm_id() {
return lkm_id;
}
public void setLkm_id(Long lkm_id) {
this.lkm_id = lkm_id;
}
public String getLkm_name() {
return lkm_name;
}
public void setLkm_name(String lkm_name) {
this.lkm_name = lkm_name;
}
public Long getLkm_cust_id() {
return lkm_cust_id;
}
public void setLkm_cust_id(Long lkm_cust_id) {
this.lkm_cust_id = lkm_cust_id;
}
public Integer getLkm_gender() {
return lkm_gender;
}
public void setLkm_gender(Integer lkm_gender) {
this.lkm_gender = lkm_gender;
}
public String getLkm_phone() {
return lkm_phone;
}
public void setLkm_phone(String lkm_phone) {
this.lkm_phone = lkm_phone;
}
public String getLkm_mobile() {
return lkm_mobile;
}
public void setLkm_mobile(String lkm_mobile) {
this.lkm_mobile = lkm_mobile;
}
public String getLkm_email() {
return lkm_email;
}
public void setLkm_email(String lkm_email) {
this.lkm_email = lkm_email;
}
public String getLkm_qq() {
return lkm_qq;
}
public void setLkm_qq(String lkm_qq) {
this.lkm_qq = lkm_qq;
}
public String getLkm_position() {
return lkm_position;
}
public void setLkm_position(String lkm_position) {
this.lkm_position = lkm_position;
}
public String getLkm_memo() {
return lkm_memo;
}
public void setLkm_memo(String lkm_memo) {
this.lkm_memo = lkm_memo;
}
public Customer getCustomer() {
return customer;
}
public void setCustomer(Customer customer) {
this.customer = customer;
}
@Override
public String toString() {
return "LinkMan{" +
"lkm_id=" + lkm_id +
", lkm_name='" + lkm_name + '\'' +
", lkm_cust_id=" + lkm_cust_id +
", lkm_gender=" + lkm_gender +
", lkm_phone='" + lkm_phone + '\'' +
", lkm_mobile='" + lkm_mobile + '\'' +
", lkm_email='" + lkm_email + '\'' +
", lkm_qq='" + lkm_qq + '\'' +
", lkm_position='" + lkm_position + '\'' +
", lkm_memo='" + lkm_memo + '\'' +
'}';
}
}
LinkMan.hbm.xml映射文件
- 2.2.2 测试:
@Test
/**
* 一对多练习
*/
public void demo02(){
Session currentSission = HibernateUtil.getCurrentSission();
Transaction transaction = currentSission.beginTransaction();
//1。创建个客户
Customer customer01 = new Customer();
customer01.setCust_name("练习王");
//2。创建对应的联系人信息
LinkMan linkMan01 = new LinkMan();
linkMan01.setLkm_name("王子一");
LinkMan linkMan02 = new LinkMan();
linkMan02.setLkm_name("王子二");
//3。建立具体人员之间的联系关系
Set linkMans = customer01.getLinkMans();
linkMans.add(linkMan01);
linkMans.add(linkMan02);
linkMan01.setCustomer(customer01);
linkMan02.setCustomer(customer01);
currentSission.save(customer01);
currentSission.save(linkMan01);
currentSission.save(linkMan02);
//4.进行事务的提交
transaction.commit();
}
测试过程中Hibernate执行的语句:
Hibernate:
insert
into
cst_customer
(cust_name, cust_source, cust_industry, cust_level, cust_phone, cust_mobile)
values
(?, ?, ?, ?, ?, ?)
Hibernate:
insert
into
cst_linkman
(lkm_name, lkm_gender, lkm_phone, lkm_mobile, lkm_email, lkm_qq, lkm_position, lkm_memo, lkm_cust_id)
values
(?, ?, ?, ?, ?, ?, ?, ?, ?)
Hibernate:
insert
into
cst_linkman
(lkm_name, lkm_gender, lkm_phone, lkm_mobile, lkm_email, lkm_qq, lkm_position, lkm_memo, lkm_cust_id)
values
(?, ?, ?, ?, ?, ?, ?, ?, ?)
Hibernate:
update
cst_linkman
set
lkm_cust_id=?
where
lkm_id=?
Hibernate:
update
cst_linkman
set
lkm_cust_id=?
where
lkm_id=?
Process finished with exit code 0
以上的代码我们可以发现建立关系是双向的,即客户关联了联系人,同时联系人也关联了客户。接下来做这么个测试,既然我们已经建立了双向的维护关系,那么我们只保存客户或只保存联系人是否可以将数据保存成功呢?
测试代码如下
@Test
public void demo2(){
Session currentSission = HibernateUtil.getCurrentSission();
Transaction transaction = currentSission.beginTransaction();
//1.建立客户
Customer customer = new Customer();
customer.setCust_name("王百万");
//2.建立联系人
LinkMan linkMan01 = new LinkMan();
linkMan01.setLkm_name("张千万");
LinkMan linkMan02 = new LinkMan();
linkMan02.setLkm_name("李千万");
//4.双向联系关系的维护
customer.getLinkMans().add(linkMan01);
customer.getLinkMans().add(linkMan02);
linkMan01.setCustomer(customer);
linkMan02.setCustomer(customer);
//5.进行数据保存
currentSission.save(customer);
transaction.commit();
}
异常信息如下:
java.lang.IllegalStateException: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.seapp.hibernate.domain.LinkMan
at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:151)
at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:181)
at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:188)
at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1490)
at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:515)
at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:3355)
at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:2526)
at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:473)
at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:178)
at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.access$300(JdbcResourceLocalTransactionCoordinatorImpl.java:39)
at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:271)
at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:104)
at com.seapp.hibernate.test.HibernateDemo02.demo2(HibernateDemo02.java:71)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.seapp.hibernate.domain.LinkMan
at org.hibernate.engine.internal.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:350)
at org.hibernate.type.EntityType.getIdentifier(EntityType.java:495)
at org.hibernate.type.EntityType.nullSafeSet(EntityType.java:280)
at org.hibernate.persister.collection.AbstractCollectionPersister.writeElement(AbstractCollectionPersister.java:926)
at org.hibernate.persister.collection.AbstractCollectionPersister.recreate(AbstractCollectionPersister.java:1349)
at org.hibernate.persister.collection.OneToManyPersister.recreate(OneToManyPersister.java:186)
at org.hibernate.action.internal.CollectionRecreateAction.execute(CollectionRecreateAction.java:50)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:604)
at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:478)
at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:356)
at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1484)
... 31 more
这样操作显然不行,无论从那一方保存都会出现同样的异常:瞬时对象异常,一个持久态对象关联了一个瞬时态对象。那如果我们想只保存一个方向应该如何操作呢,从而引入Hibernate的级联操作
第三节:Hiberanate的级联操作
级联操作是指当主控方执行保存、更新或者删除操作时,其关联对象(被控方)也执行相同的操作。在映射文件中通过对cascade属性的设置来控制是否对关联对象采用级联操作。级联操作对各种关联关系都是有效的。
级联是有方向性的,所谓的方向性指的是,在保存一的一方级联多的一方和在保存多的一方级联一的一方。配置cascade后,操作其中一方则关联一方也执行相应的操作。
级联查询
@Test
//导航查询
public void findTest(){
Session currentSission = HibernateUtil.getCurrentSission();
Transaction transaction = currentSission.beginTransaction();
Customer customer = currentSission.load(Customer.class, 6L);
System.out.println(customer.toString());
transaction.commit();
}
查询结果:
Hibernate:
select
customer0_.cust_id as cust_id1_0_0_,
customer0_.cust_name as cust_nam2_0_0_,
customer0_.cust_source as cust_sou3_0_0_,
customer0_.cust_industry as cust_ind4_0_0_,
customer0_.cust_level as cust_lev5_0_0_,
customer0_.cust_phone as cust_pho6_0_0_,
customer0_.cust_mobile as cust_mob7_0_0_
from
cst_customer customer0_
where
customer0_.cust_id=?
Hibernate:
select
linkmans0_.lkm_cust_id as lkm_cus10_1_0_,
linkmans0_.lkm_id as lkm_id1_1_0_,
linkmans0_.lkm_id as lkm_id1_1_1_,
linkmans0_.lkm_name as lkm_name2_1_1_,
linkmans0_.lkm_gender as lkm_gend3_1_1_,
linkmans0_.lkm_phone as lkm_phon4_1_1_,
linkmans0_.lkm_mobile as lkm_mobi5_1_1_,
linkmans0_.lkm_email as lkm_emai6_1_1_,
linkmans0_.lkm_qq as lkm_qq7_1_1_,
linkmans0_.lkm_position as lkm_posi8_1_1_,
linkmans0_.lkm_memo as lkm_memo9_1_1_,
linkmans0_.lkm_cust_id as lkm_cus10_1_1_
from
cst_linkman linkmans0_
where
linkmans0_.lkm_cust_id=?
Customer{cust_id=6, cust_name='王百万', cust_source='null', cust_industry='null', cust_level='null', cust_phone='null', cust_mobile='null', linkMans=[LinkMan{lkm_id=6, lkm_name='张千万', lkm_cust_id=null, lkm_gender=null, lkm_phone='null', lkm_mobile='null', lkm_email='null', lkm_qq='null', lkm_position='null', lkm_memo='null'}, LinkMan{lkm_id=5, lkm_name='李千万', lkm_cust_id=null, lkm_gender=null, lkm_phone='null', lkm_mobile='null', lkm_email='null', lkm_qq='null', lkm_position='null', lkm_memo='null'}]}
Process finished with exit code 0
第四节:双向关联产生多余的SQL语句的问题解决
因为双向维护了关系,而且持久态对象可以自动更新数据库,更改客户的时候会修改一次外键,更新联系人的时候同样也会修改一次外键。这样就会产生多余的SQL,那么如何解决呢?
其实解决额方法很简单:只需要将一方放弃外键维护权即可 。也就是说关系不是双方维护的,只需要交给某一方去维护就可以了。通常我们都是交给多的一方去维护的,为什么呢?因为多的一方才是维护关系最好的地方。举个例子:一个老师对应多个学生,一个学生对应多个老师,这是典型的一对多,那么一个老师如果要记住所有学生的名字很难,但如果让每个学生记住老师的名字应该不难。其实就是这个道理。所以在一对多中,一的一方都会放弃对外界的维护权(关系的维护)。
如果想让一的一方放弃外键的维护权,只需要进行如下的配置:
inverse的默认值是false,代表不放弃对外键的维护权,配置值为true,代表放弃了外键的维护权。
第五节:Hibernate中多对多映射关系的建立。
- 5.1 表结构的创建
用户表
CREATE TABLE `sys_user` (
`user_id` bigint(32) NOT NULL AUTO_INCREMENT COMMENT '用户id',
`user_code` varchar(32) DEFAULT NULL COMMENT '用户账号',
`user_name` varchar(64) DEFAULT NULL COMMENT '用户名称',
`user_password` varchar(32) DEFAULT NULL COMMENT '用户密码',
`user_state` char(1) DEFAULT NULL COMMENT '1:正常,0:暂停',
PRIMARY KEY (`user_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
角色表:
CREATE TABLE `sys_role` (
`role_id` bigint(32) NOT NULL AUTO_INCREMENT,
`role_name` varchar(32) NOT NULL COMMENT '角色名称',
`role_memo` varchar(128) DEFAULT NULL COMMENT '备注',
PRIMARY KEY (`role_id`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
中间表:
CREATE TABLE `sys_user_role` (
`role_id` bigint(32) NOT NULL COMMENT '角色id',
`user_id` bigint(32) NOT NULL COMMENT '用户id',
PRIMARY KEY (`role_id`,`user_id`),
KEY `FK_user_role_user_id` (`user_id`),
CONSTRAINT `FK_user_role_role_id` FOREIGN KEY (`role_id`) REFERENCES `sys_role` (`role_id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `FK_user_role_user_id` FOREIGN KEY (`user_id`) REFERENCES `sys_user` (`user_id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
- 5.2 实体的创建
package com.seapp.hibernate.domain;
import java.util.HashSet;
import java.util.Set;
/**
* user实体类
*/
public class User {
private Long user_id;
private String user_code;
private String user_name;
private String user_password;
private String user_state;
private Set roles = new HashSet<>();
public Long getUser_id() {
return user_id;
}
public void setUser_id(Long user_id) {
this.user_id = user_id;
}
public String getUser_code() {
return user_code;
}
public void setUser_code(String user_code) {
this.user_code = user_code;
}
public String getUser_name() {
return user_name;
}
public void setUser_name(String user_name) {
this.user_name = user_name;
}
public String getUser_password() {
return user_password;
}
public void setUser_password(String user_password) {
this.user_password = user_password;
}
public String getUser_state() {
return user_state;
}
public void setUser_state(String user_state) {
this.user_state = user_state;
}
public Set getRoles() {
return roles;
}
public void setRoles(Set roles) {
this.roles = roles;
}
}
/************************Hibernate中User的映射表**********************************/
/**********************************************************/
package com.seapp.hibernate.domain;
import java.util.HashSet;
import java.util.Set;
/**
* 角色实体类
*/
public class Role {
private Long role_id;
private String role_name;
private String role_memo;
private Set users = new HashSet<>();
public Long getRole_id() {
return role_id;
}
public void setRole_id(Long role_id) {
this.role_id = role_id;
}
public String getRole_name() {
return role_name;
}
public void setRole_name(String role_name) {
this.role_name = role_name;
}
public String getRole_memo() {
return role_memo;
}
public void setRole_memo(String role_memo) {
this.role_memo = role_memo;
}
public Set getUsers() {
return users;
}
public void setUsers(Set users) {
this.users = users;
}
}
/*************************Hibernate中role的映射关系*********************************/
在多对多的保存操作中,如果进行了双向维护关系,就必须有一方放弃外键维护权。一般由被动方放弃,用户主动选择角色,角色是被选择的,所以一般角色要放弃外键的维护权。
之前学习过的一对多的级联保存及删除在多对多中依然生效,通过配置cascade的属性来实现。
多对多的关系主要是靠中间表来维护的,那么在Hibernate中多对多主要是靠关联的集合来维护的,所以在多对多我们只需要关心如何操作集合即可。