MyBatis基础篇之使用association解决一对一关联查询

一、前言

上一章节<<MyBatis基础篇之结果映射ResultMap级联属性封装>>，我们通过级联属性的方式，将角色中的数据查出来封装到用户User的role属性里面，我们还有另外一种方式来实现数据的封装。接下来我们将介绍一下association标签的相关用法，包括嵌套查询和分段查询两种方式。

二、案例

♦参照前面介绍的相关章节，如<<MyBatis基础篇之简单入门>>等，搭建工程MyBatisAssociationDemo

♦在MySQL数据库新建两张数据库表t_user和t_role，并插入若干条数据

CREATE TABLE t_user (
id int(10) NOT NULL AUTO_INCREMENT,
loginId varchar(20) DEFAULT NULL,
userName varchar(100) DEFAULT NULL,
roleId int(10),
note varchar(255) DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;

INSERT INTO t_user(loginId,userName,roleId,note) VALUES ('queen', '奎恩', 1, '专门负责提鞋的。。。');
INSERT INTO t_user(loginId,userName,roleId,note) VALUES ('king', '金狮子', 2, '磁性果实能力');
INSERT INTO t_user(loginId,userName,roleId,note) VALUES ('Lucy', '路西', 3, '打败多弗朗明哥。。。');
=========================================================
CREATE TABLE t_role (
id int(10) NOT NULL AUTO_INCREMENT,
roleName varchar(20) DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;

INSERT INTO t_role(roleName) VALUES ('自定义角色');
INSERT INTO t_role(roleName) VALUES ('前海贼');
INSERT INTO t_role(roleName) VALUES ('未来海贼王');

[/code]

♦新建JavaBean类 User

/**
* @since 2017-08-08
* @author queen
* 定义一个Java类
*
*/
public class User {
// ID，唯一性
private int id;
// 登录ID
private String loginId;
// 用户名
private String userName;
// 角色
private Role role;
// 备注
private String note;

public User(){

}

public User(int id, String loginId, String userName, Role role, String note) {
this.id = id;
this.loginId = loginId;
this.userName = userName;
this.role = role;
this.note = note;
}

public int getId() {
return id;
}

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

public String getLoginId() {
return loginId;
}

public void setLoginId(String loginId) {
this.loginId = loginId;
}

public String getUserName() {
return userName;
}

public void setUserName(String userName) {
this.userName = userName;
}

public Role getRole() {
return role;
}

public void setRole(Role role) {
this.role = role;
}

public String getNote() {
return note;
}

public void setNote(String note) {
this.note = note;
}

@Override
public String toString() {
return "User [id=" + id + ", loginId=" + loginId + ", userName="
+ userName + ", role=" + role + ", note=" + note + "]";
}

}

[/code]

♦新建JavaBean类 Role

/**
* @since 2017-08-08
* @author queen
* 定义一个Java类Role
*
*/
public class Role {
private int id;
private String roleName;

public int getId() {
return id;
}

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

public String getRoleName() {
return roleName;
}

public void setRoleName(String roleName) {
this.roleName = roleName;
}

public Role(){

}

public Role(int id, String roleName) {
this.id = id;
this.roleName = roleName;
}

@Override
public String toString() {
return "Role [id=" + id + ", roleName=" + roleName + "]";
}

}

[/code]

♦新建接口类UserMapper.java，增加接口方法getUserById

import com.queen.mybatis.bean.User;
/**
* @since 2017-08-08
* @author queen
* 定义一个UserMapper接口
*
*/
public interface UserMapper {

/**
* 根据ID查询User信息，返回一条记录User
* @param id
* @return
*/
public User getUserById(int id);

}

[/code]

♦核心配置文件db.properties，log4j.properties，mybatis-config.xml可以直接从<<MyBatis基础篇之简单入门>>拷贝过来

主要的核心文件都已写好，接下来我们将分步介绍嵌套查询和分段查询两种方式

♦首先我们来看一下嵌套查询，UserMapper.xml配置文件如下：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.queen.mybatis.mapper.UserMapper">

<resultMap type="com.queen.mybatis.bean.User" id="userResultMap">
<id property="id" column="id"/>
<result property="loginId" column="loginId" />
<result property="userName" column="userName"/>
<result property="note" column="note"/>
<!--assocication可以指定联合的JavaBean对象
property="role"指定哪个属性是联合的对象
javaType:指定这个属性对象的类型
-->
<association property="role" javaType="com.queen.mybatis.bean.Role">
<id column="role_id" property="id"/>
<result column="roleName" property="roleName"/>
</association>
</resultMap>

<select id="getUserById" resultMap="userResultMap">
select m.id id, m.loginId loginId, m.userName userName, m.roleId roleId,m.note note, n.id role_id, n.roleName roleName
from t_user m left join t_role n on m.roleId=n.id
where m.id=#{id}
</select>

</mapper>

[/code]

♦新增测试类MyBatisTest，添加测试方法testGetUserByAssocication

@Test
public void testGetUserByAssocication() throws IOException {
SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
SqlSession openSession = sqlSessionFactory.openSession();
try {
UserMapper mapper = openSession.getMapper(UserMapper.class);
User user = mapper.getUserById(1);
System.out.println(user);
System.out.println(user.getRole());
} finally {
openSession.close();
}
}

[/code]

♦测试，控制台打印结果如下：

2017-08-08 22:46:52,470 [main] [com.queen.mybatis.mapper.UserMapper.getUserById]-[DEBUG] ==>  Preparing: select m.id id, m.loginId loginId, m.userName userName, m.roleId roleId,m.note note, n.id role_id, n.roleName roleName from t_user m left join t_role n on m.roleId=n.id where m.id=?
2017-08-08 22:46:52,558 [main] [com.queen.mybatis.mapper.UserMapper.getUserById]-[DEBUG] ==> Parameters: 1(Integer)
2017-08-08 22:46:52,597 [main] [com.queen.mybatis.mapper.UserMapper.getUserById]-[DEBUG] <==      Total: 1
User [id=1, loginId=queen, userName=奎恩, role=Role [id=1, roleName=自定义角色], note=专门负责提鞋的。。。]
Role [id=1, roleName=自定义角色]

[/code]这样我们可以通过association也能够实现关联属性的查询，使用association定义关联的单个对象的封装结果。接下来我们介绍第二种：使用association进行分步查询，那么分步查询是什么意思呢？用我们上述例子，即我们查完User表数据后，我们在查询Role表数据。分多条SQL执行。废话不多说了，直接上代码

♦定义一个接口类RoleMapper，新增接口方法getRoleById

import com.queen.mybatis.bean.Role;

/**
* @since 2017-08-08
* @author queen
* 定义一个RoleMapper接口
*
*/
public interface RoleMapper {
/**
* 根据ID查询Role信息，返回一条记录Role
* @param id
* @return
*/
public Role getRoleById(int id);
}

[/code]

♦定义一个RoleMapper.xml文件，配置如下：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.queen.mybatis.mapper.RoleMapper">
<select id="getRoleById" resultType="com.queen.mybatis.bean.Role">
select id, roleName from t_role where id=#{id}
</select>
</mapper>

[/code]

♦修改UserMapper的Java文件，新增一个方法getUserByIdStep

/**
* 根据ID查询User信息，返回一条记录User，分步查询
* @param id
* @return
*/
public User getUserByIdStep(int id);

[/code]

♦修改UserMapper.xml文件，增加如下配置

<resultMap type="com.queen.mybatis.bean.User" id="userResultMapStep">
<id property="id" column="id"/>
<result property="loginId" column="loginId" />
<result property="userName" column="userName"/>
<result property="note" column="note"/>
<!-- association定义关联对象的封装规则
select:表明当前属性是调用select指定的方法查询出结果
column:指定将那一列的值传递给这个方法
整个流程：使用Select指定的方法(传入column指定的这列参数的值)查出对象，并封装给property指定的属性
-->
<association property="role" select="com.queen.mybatis.mapper.RoleMapper.getRoleById"
column="roleId">
</association>
</resultMap>
<select id="getUserByIdStep" resultMap="userResultMapStep">
select * from t_user where id=#{id}
</select>

[/code]

♦修改测试类MyBatisTest，添加测试方法testGetUserByAssocicationStep

@Test
public void testGetUserByAssocicationStep() throws IOException {
SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
SqlSession openSession = sqlSessionFactory.openSession();
try {
UserMapper mapper = openSession.getMapper(UserMapper.class);
User user = mapper.getUserByIdStep(1);
System.out.println(user);
System.out.println(user.getRole());
} finally {
openSession.close();
}
}

[/code]

♦测试方法，控制台打印结果如下：

org.apache.ibatis.exceptions.PersistenceException:
### Error querying database.  Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.queen.mybatis.mapper.RoleMapper.getRoleById
### The error may exist in UserMapper.xml
### The error may involve com.queen.mybatis.mapper.UserMapper.getUserByIdStep
### The error occurred while handling results
### SQL: select * from t_user where id=?
### Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.queen.mybatis.mapper.RoleMapper.getRoleById
at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:26)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:111)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:102)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:66)

[/code]结果事与愿违，报错了，这是为什么呢？注意我们一定要将新增的RoleMapper.xml映射文件配置到mybatis-config.xml文件中，配置如下：

<mappers>
<mapper resource="UserMapper.xml" />
<mapper resource="RoleMapper.xml" />
</mappers>

重新测试一下

2017-08-08 23:26:04,799 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==>  Preparing: select * from t_user where id=?
2017-08-08 23:26:04,853 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==> Parameters: 1(Integer)
2017-08-08 23:26:04,909 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] ====>  Preparing: select id, roleName from t_role where id=?
2017-08-08 23:26:04,909 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] ====> Parameters: 1(Integer)
2017-08-08 23:26:04,911 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] <====      Total: 1
2017-08-08 23:26:04,911 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] <==      Total: 1
User [id=1, loginId=queen, userName=奎恩, role=Role [id=1, roleName=自定义角色], note=专门负责提鞋的。。。]
Role [id=1, roleName=自定义角色]

从上述打印结果，可以看到控制台打印了两段SQL语句，一句用来查询User信息，另外一句用来查询Role信息，实现了分步查询的效果，且控制台打印结果正常。

♦完整工程结构目录如下：

至此，我们关于MyBatis基础篇之使用association解决一对一关联查询介绍完毕。博客地址：http://www.marsitman.com/mybatis/mybatis-assocication.html