MyBatis应用开发(15)映射之一对多关联

Posted 2020-09-12

1.1.1. 一对多关联

一个Person可以拥有一个或多个Address，而每个Address对应于唯一一个Person，因此Person和Address之间存在一对多的关系。本文介绍使用MyBatis如何进行基于List的一对多映射。

 

Person类：

/**   

* @Title: Person.java

* @Package com.test.mybatis3.pojo

* @Description:

* @author http://www.cnblogs.com/coe2coe/

* @date 2017年4月9日 下午3:21:32

* @version V1.0   

*/

package com.test.mybatis3.pojo;

 

import java.util.List;

 

/**

* @ClassName: Person

* @Description:

* @author http://www.cnblogs.com/coe2coe/

* @date 2017年4月9日 下午3:21:32

*  

*/

public class Person {

 

private  String id;

private  String name;

private  int    status;

 

//用于满足POJO要求的无参数构造函数。

public Person()

{

}

 

 

public String getId() {

return id;

}

public String getName() {

return name;

}

public int getStatus() {

return status;

}

 

public void setId(String id) {

this.id = id;

}

 

public void setName(String name) {

this.name = name;

}

 

public void setStatus(int status) {

this.status = status;

}

 

@Override

public String toString() {

return "Person [id=" + id + ", name=" + name + ", status=" + status +"]";

}

 

 

 

private List<Address>  addressList;

 

public List<Address> getAddressList() {

return addressList;

}

 

 

public void setAddressList(List<Address> addressList) {

this.addressList = addressList;

}

}

 

 

 

 

Address类：

/**   

* @Title: Address.java

* @Package com.test.mybatis3.pojo

* @Description:

* @author http://www.cnblogs.com/coe2coe/

* @date 2017年4月17日 下午10:56:26

* @version V1.0   

*/

package com.test.mybatis3.pojo;

 

/**

* @ClassName: Address

* @Description:

* @author http://www.cnblogs.com/coe2coe/

* @date 2017年4月17日 下午10:56:26

*  

*/

public class Address {

 

private int  addressid;

private String personid;

private String addressname;

public int getAddressid() {

return addressid;

}

public void setAddressid(int addressid) {

this.addressid = addressid;

}

public String getPersonid() {

return personid;

}

public void setPersonid(String personid) {

this.personid = personid;

}

public String getAddressname() {

return addressname;

}

public void setAddressname(String addressname) {

this.addressname = addressname;

}

@Override

public String toString() {

return "Address [addressid=" + addressid + ", personid=" + personid + ", addressname=" + addressname + "]";

}

 

 

private Person  person;

public Person getPerson() {

return person;

}

public void setPerson(Person person) {

this.person = person;

}

}

 

 

Mapper配置文件：

有两种映射方式：

(1)一种是使用一条JOIN语句将Person和对应的Address集合全部查询出来。

(2)另外一种是先查询出Person列表，再根据personid使用SELECT子查询查询出Address集合。

 

使用JOIN语句的Mapper配置方式：

<!-- 查询Person和对应的Address集合 -->

<select id="findPersonAndAddress" resultMap="personResultMap" >

select a.* ,b.* from t_person a

left outer join t_address b on (a.id = b.personid)  

order by a.id asc , b.addressid asc

</select>

 

<!-- 将t_person映射到Person类 -->

<resultMap type="com.test.mybatis3.pojo.Person" id="personResultMap">

<id property="id" column="id" />

<result  property="name" column="name" />

<result  property="status" column="status" />

 

<!-- 将该Person对应的t_address记录集映射到addressList属性 -->

<collection property="addressList" javaType="java.util.ArrayList" resultMap="addressResultMap" />

 

</resultMap>

 

<!-- 将t_address记录映射到Address类 -->

<resultMap type="com.test.mybatis3.pojo.Address" id="addressResultMap">

<id property="addressid" column="addressid" />

<result property="personid" column="personid" />

<result property="addressname" column="addressname" />

</resultMap>

 

 

使用嵌套查询的配置方式：

<!-- 查询Person和对应的Address集合 -->

<!-- 计划采用select子查询的方式来获取Address集合，因此此处仅查询person -->

<select id="findPersonAndAddress" resultMap="personResultMap" >

select * from t_person order by id asc

</select>

 

<!-- 将t_person映射到Person类 -->

<resultMap type="com.test.mybatis3.pojo.Person" id="personResultMap">

<id property="id" column="id" />

<result  property="name" column="name" />

<result  property="status" column="status" />

 

<!-- 将该Person对应的t_address记录集映射到addressList属性 -->

<!-- select:一个子查询的id -->

<collection select="findAddressByPersonid"

            column="id" property="addressList"

            javaType="java.util.ArrayList"  

            />

 

</resultMap>

 

<!-- 将t_address记录映射到Address类 -->

<resultMap type="com.test.mybatis3.pojo.Address" id="addressResultMap">

<id property="addressid" column="addressid" />

<result property="personid" column="personid" />

<result property="addressname" column="addressname" />

</resultMap>

 

<!-- 根据personid查询Address集合 -->

<select parameterType="string" resultMap="addressResultMap" id="findAddressByPersonid" >

select * from t_address where personid=#{id}

</select>

 

 

 

 

 

Mapper接口的方法：

 

public interface PersonMapper { 

List<Person> findPersonAndAddress() throws Exception;

}

 

 

单元测试代码如下：

//打开Session。

session = sessionBuilder.openSession();

 

//找到MyBatis自动实现的PersonMapper接口的代理对象。

PersonMapper  personMapper = session.getMapper(PersonMapper.class);

 

//查询Person和相应的Address集合。

List<Person>  persons = personMapper.findPersonAndAddress();

for(Person person : persons){

System.out.println(person);

for(Address address: person.getAddressList()){

System.out.println(address);

}

System.out.println("----------------------");

}

 

 

 

 

运行结果如下：

 

0    [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - ==>  Preparing: select a.* ,b.* from t_person a left outer join t_address b on (a.id = b.personid) order by a.id asc , b.addressid asc

42   [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - ==> Parameters:

68   [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==    Columns: id, name, status, addressid, personid, addressname

68   [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==        Row: lisi, li si, 0, 4, lisi, road1

72   [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==        Row: lisi, li si, 0, 5, lisi, road2

74   [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==        Row: lisi, li si, 0, 6, lisi, road3

74   [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==        Row: zhangsan, zhang san, 0, 1, zhangsan, street1

76   [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==        Row: zhangsan, zhang san, 0, 2, zhangsan, street2

76   [main] TRACE com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==        Row: zhangsan, zhang san, 0, 3, zhangsan, street3

76   [main] DEBUG com.test.mybatis3.mapper.PersonMapper.findPersonAndAddress  - <==      Total: 6

Person [id=lisi, name=li si, status=0]

Address [addressid=4, personid=lisi, addressname=road1]

Address [addressid=5, personid=lisi, addressname=road2]

Address [addressid=6, personid=lisi, addressname=road3]

----------------------

Person [id=zhangsan, name=zhang san, status=0]

Address [addressid=1, personid=zhangsan, addressname=street1]

Address [addressid=2, personid=zhangsan, addressname=street2]

Address [addressid=3, personid=zhangsan, addressname=street3]

----------------------