mybatis关联查询之一对多，多对一，以及多对多

一、使用IDEA新建maven工程

二、引入mybatis以及相关的jar

<!--版本仅供参考-->
<dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
        </dependency>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.5</version>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.44</version>
        </dependency>
    </dependencies>

三、建表

　　权限管理需求的介绍，一个用户拥有若干角色，一个角色拥有若干权限，权限就是对某个资源的增删改查，这样就构成了用户-角色-权限之间的模型，在这种关系中，用户与角色，角色与权限之间一般是多对多的关系

　建表sql：

create table sys_user(
    id int not null auto_increment comment '用户ID',
    user_name varchar(32) comment '用户名',
    password varchar(50) comment '用户密码',
    usr_info varchar(50) comment '用户简介',
    head_img blob comment '头像',
    create_time datetime comment '创建时间',
    primary key(id)
);
alter table sys_user comment  '用户表';

create table sys_role(
    id int not null auto_increment comment  '角色ID',
    role_name varchar(50) comment '角色名称',
    enabled  int comment '有效标志',
    create_by  bigint comment '创建人',
    create_time datetime comment '创建时间',
    primary key(id)    
);
alter table sys_role comment '角色表';

create table sys_privilege(
    id int not null auto_increment  comment '权限ID',
    privilege_name varchar(50) comment '权限名称',
    privilege_url varchar(50) comment '权限URL',
    primary key(id)
);
alter table sys_privilege comment '权限表';

create table sys_user_role(
    user_id bigint not null comment '用户ID',
    role_id bigint not null comment '角色ID'
);
alter table sys_user comment '用户角色';
create table sys_role_privilege(
    role_id bigint not null comment '角色ID',
    privilege_id  bigint  not null comment '权限ID'
);
alter table sys_role_privilege comment '角色权限表';

给表添加外键

--   给用户角色表的user_id添加外键 
alter table sys_user_role add constraint fk_uru foreign key(user_id) references sys_user(id);
--   给用户角色表的role_id 添加外键 
alter table sys_user_role add constraint fk_sysrur foreign key(role_id) references sys_role(id);
--    给角色权限表的role_id添加外键 
alter table sys_role_privilege add constraint fk_sysrpp foreign key(privilege_id) references sys_privilege(id);
--    给角权限的privilege_id 添加外键 
alter table sys_role_privilege add constraint fk_sysrpr foreign key(role_id) references sys_role(role_id);

插入数据

insert into sys_user(user_name,password,user_info,head_img,create_time )values('Jordan','123','管理员',null,'2018-1-22 12:02:01'),
('Alice','456','测试人员',null,'2018-1-22 12:02:01'),
('Alex','789','开发人员',null,'2018-1-22 12:02:01'),
('James','012','运维人员',null,'2018-1-22 12:02:01')
insert into sys_user_role(role_name,enabled,create_by,create_time) values('管理员',1,'1','2018-01-02'),values('普通用户',1,'1','2018-01-02');
insert into sys_privilege(privilege_name,privilege_url) values('用户管理','/users'),('角色管理','/roles'),('系统维护','/system'),('日志管理','/logs');
insert into sys_user_role values(1,1),(2,2),(3,2),(4,2);
insert into sys_role_privilege values(1,1),(2,3),(2,4);

四、使用逆向工程生成entity以及mapper接口和mapper文件

　　①：创建mybatis-config.xml文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration>
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
    <typeAliases>
        <package name="com.jordan.mybatis.entity"></package>
        </typeAliases>
    <environments default="defaultEnv">
        <environment id="defaultEnv">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="username" value="root"></property>
                <property name="password" value="123456"></property>
                <property name="url" value="jdbc:mysql://localhost:3306/mybatis"></property>
                <property name="driver" value="com.mysql.jdbc.Driver"></property>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <package name="com.jordan.mybatis.mapper"></package>
    </mappers>
</configuration>

　　②：SysUser.java

package com.jordan.mybatis.entity;

import java.util.Arrays;
import java.util.Date;
import java.util.List;

public class SysUser {
    private Long id;

    private String userName;

    private String password;

    private Date createTime;

    private String userInfo;

    private byte[] headImg;

    private SysRole sysRole;

    private List<SysRole> sysRoleList;

    public List<SysRole> getSysRoleList() {
        return sysRoleList;
    }

    public void setSysRoleList(List<SysRole> sysRoleList) {
        this.sysRoleList = sysRoleList;
    }

    public SysRole getSysRole() {
        return sysRole;
    }

    public void setSysRole(SysRole sysRole) {
        this.sysRole = sysRole;
    }

    public Long getId() {
        return id;
    }

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

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName == null ? null : userName.trim();
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password == null ? null : password.trim();
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }


    public String getUserInfo() {
        return userInfo;
    }

    public void setUserInfo(String userInfo) {
        this.userInfo = userInfo == null ? null : userInfo.trim();
    }

    public byte[] getHeadImg() {
        return headImg;
    }

    public void setHeadImg(byte[] headImg) {
        this.headImg = headImg;
    }

    @Override
    public String toString() {
        return "SysUser{" +
                "id=" + id +
                ", userName='" + userName + '\'' +
                ", password='" + password + '\'' +
                ", createTime=" + createTime +
                ", userInfo='" + userInfo + '\'' +
                ", headImg=" + Arrays.toString(headImg) +
                ", sysRole=" + sysRole +
                ", sysRoleList=" + sysRoleList +
                '}';
    }
}

　　③：SysRole.java

package com.jordan.mybatis.entity;

import java.util.Arrays;
import java.util.Date;
import java.util.List;

public class SysUser {
    private Long id;

    private String userName;

    private String password;

    private Date createTime;

    private String userInfo;

    private byte[] headImg;

    private SysRole sysRole;

    private List<SysRole> sysRoleList;

    public List<SysRole> getSysRoleList() {
        return sysRoleList;
    }

    public void setSysRoleList(List<SysRole> sysRoleList) {
        this.sysRoleList = sysRoleList;
    }

    public SysRole getSysRole() {
        return sysRole;
    }

    public void setSysRole(SysRole sysRole) {
        this.sysRole = sysRole;
    }

    public Long getId() {
        return id;
    }

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

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName == null ? null : userName.trim();
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password == null ? null : password.trim();
    }

    public Date getCreateTime() {
        return createTime;
    }

    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }


    public String getUserInfo() {
        return userInfo;
    }

    public void setUserInfo(String userInfo) {
        this.userInfo = userInfo == null ? null : userInfo.trim();
    }

    public byte[] getHeadImg() {
        return headImg;
    }

    public void setHeadImg(byte[] headImg) {
        this.headImg = headImg;
    }

    @Override
    public String toString() {
        return "SysUser{" +
                "id=" + id +
                ", userName='" + userName + '\'' +
                ", password='" + password + '\'' +
                ", createTime=" + createTime +
                ", userInfo='" + userInfo + '\'' +
                ", headImg=" + Arrays.toString(headImg) +
                ", sysRole=" + sysRole +
                ", sysRoleList=" + sysRoleList +
                '}';
    }
}

　　④：SysPrivilege.java

package com.jordan.mybatis.entity;

public class SysPrivilege {
    private Long id;

    private String privilegeName;

    private String privilegeUrl;

    public Long getId() {
        return id;
    }

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

    public String getPrivilegeName() {
        return privilegeName;
    }

    public void setPrivilegeName(String privilegeName) {
        this.privilegeName = privilegeName == null ? null : privilegeName.trim();
    }

    public String getPrivilegeUrl() {
        return privilegeUrl;
    }

    public void setPrivilegeUrl(String privilegeUrl) {
        this.privilegeUrl = privilegeUrl == null ? null : privilegeUrl.trim();
    }

    @Override
    public String toString() {
        return "SysPrivilege{" +
                "id=" + id +
                ", privilegeName='" + privilegeName + '\'' +
                ", privilegeUrl='" + privilegeUrl + '\'' +
                '}';
    }
}

　　⑤:SysUserMapper

public interface SysUserMapper {
    public List<SysUser> getUserRolePrivilege();
}

　　⑥：SysUserMapper.xml

<?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.jordan.mybatis.mapper.SysUserMapper">
    <resultMap id="getUserRolePrivilege" type="SysUser">
        <id property="id" column="id"></id>
        <result property="userName" column="user_name"></result>
        <result property="password" column="password"></result>
        <result property="userInfo" column="user_info"></result>
        <result property="createTime" column="create_time"></result>
        <collection property="sysRoleList" ofType="SysRole">
            <id property="id" column="id"></id>
            <result property="roleName" column="role_name"></result>
            <result property="createTime" column="create_time"></result>
            <collection property="sysPrivilegeList" ofType="SysPrivilege">
                <id property="id" column="id"></id>
                <result property="privilegeName" column="privilege_name"></result>
                <result  property="privilegeUrl" column="privilege_url"></result>
            </collection>
        </collection>
    </resultMap>
    <select id="getUserRolePrivilege"  resultMap="getUserRolePrivilege">
       select a.id,a.user_name,a.password,a.user_info,b.role_name,b.create_time,c.privilege_name,c.privilege_url from sys_user a,sys_role b,
    sys_privilege c,sys_user_role d,sys_role_privilege e  where a.id = d.user_id and b.id = d.role_id and e.role_id=b.id and e.privilege_id=c.id
    </select>
</mapper>

五、在mybatis-config.xml中添加mapper接口以及mapper文件

<mappers>
        <package name="com.jordan.mybatis.mapper"></package>
</mappers>

六、创建测试类Test.java

import com.jordan.mybatis.mapper.SysUserMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;

import java.io.IOException;
import java.io.InputStream;
import java.io.Reader;

/**
 * @author Jordan
 * @create 
 * @DESCRIPTION
 */
public class Test {
    private  SqlSessionFactory sqlSessionFactory;
    @Before
    public void init() throws IOException{
        //1:读取mybatis的主配置文件
        String configFile = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(configFile);
        //2:通过sqlSessionFactoryBuilder创建一个sqlSessionFactory
        sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    }
    @org.junit.Test
    public void testGetUserRolePrivilege(){
        SqlSession sqlSession=sqlSessionFactory.openSession();
        SysUserMapper sysUserMapper=sqlSession.getMapper(SysUserMapper.class);
        System.out.println(sysUserMapper.getUserRolePrivilege().size());    //4 查询结果与数据库查询结果不匹配，发生数据覆盖问题，解决办法参考注意事项②
　　　　　sqlSession.close();

    }
}

七、注意事项：

①：idea找不到mapper文件的原因：IDEA不会编译src的java目录下的xml文件。所以添加如下代码

      <resources>
          <!-- resources文件 -->
          <resource>
              <directory>src/main/resources</directory>
              <!-- 是否被过滤,如果被过滤则无法使用 -->
              <filtering>false</filtering>
          </resource>
          <!-- java文件夹 -->
          <resource>
              <directory>src/main/java</directory>
              <!-- 引入映射文件等 -->
              <includes>
                  <include>**/*.xml</include>
                  <include>**/*.properties</include>
              </includes>
          </resource>
          <resource>
              <directory>src/main/webapp</directory>
              <includes>
                  <include>**/*.*</include>
              </includes>
          </resource>
      </resources>

②：mybatis在映射依赖的过程中，如果两个表中的主键是一样的会发生数据覆盖问题。

解决办法：　 

　　解决方式一：修改数据库表中的主键（这种方法比较麻烦）；

　　解决方式二：在查询语句中修改一个表的主键使其不一致即可；

上述代码测试结果中与数据库中的查询记录不匹配

数据库查询结果为8条记录，但是mybatis查询只有4条数据，发生数据覆盖的问题；

修改SysUserMapper.xml文件：

<?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.jordan.mybatis.mapper.SysUserMapper">
    <resultMap id="getUserRolePrivilege" type="SysUser">
        <id property="id" column="aid"></id>
        <result property="userName" column="user_name"></result>
        <result property="password" column="password"></result>
        <result property="userInfo" column="user_info"></result>
        <result property="createTime" column="create_time"></result>
        <collection property="sysRoleList" ofType="SysRole">
            <id property="id" column="bid"></id>
            <result property="roleName" column="role_name"></result>
            <result property="createTime" column="create_time"></result>
            <collection property="sysPrivilegeList" ofType="SysPrivilege">
                <id property="id" column="cid"></id>
                <result property="privilegeName" column="privilege_name"></result>
                <result property="privilegeUrl" column="privilege_url"></result>
            </collection>
        </collection>
    </resultMap>
    <select id="getUserRolePrivilege" resultMap="getUserRolePrivilege">
    select a.id,a.user_name,a.password,a.user_info,b.role_name,b.create_time,c.privilege_name,c.privilege_url from sys_user a,sys_role b,
    sys_privilege c,sys_user_role d,sys_role_privilege e  where a.id = d.user_id and b.id = d.role_id and e.role_id=b.id and e.privilege_id=c.id
    </select>
</mapper>