Mybaits（12）MyBatis级联-3

一、另一种级联

　　MyBatis还提供了另一种级联方式，它是基于sql表连接的基础上，进行再设计的，之前我们在一对一级联时简单介绍过。现在我们看如何使用。

1.定义sql，修改IEmployeeDao.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.xhbjava.dao.IEmployeeDao">



    <resultMap id="employee" type="com.xhbjava.domain.Employee">
        <id column="id" property="id" />
        <result column="real_name" property="realName" />
        <result column="sex" property="sex"
            typeHandler="com.xhbjava.typeHandler.SexTypeHandler" />
        <result column="birthday" property="birthday" />
        <result column="mobile" property="mobile" />
        <result column="email" property="email" />
        <result column="position" property="position" />
        <association property="workcard"
            javaType="com.xhbjava.domain.WorkCard" column="id">
            <id column="wc_id" property="id" />
            <result column="id" property="empId" />
            <result column="wc_real_name" property="realName" />
            <result column="wc_department" property="department" />
            <result column="wc_mobile" property="mobile" />
            <result column="wc_position" property="position" />
            <result column="wc_note" property="note" />
        </association>
        <collection property="employeeTaskList"
            ofType="com.xhbjava.domain.EmployeeTask" column="id">
            <id column="et_id" property="id" />
            <result column="id" property="empId" />
            <result column="task_name" property="taskName" />
            <result column="note" property="note" />
            <association property="task"
                javaType="com.xhbjava.domain.Task" column="et_task_id">
                <id column="t_id" property="id" />
                <result column="t_title" property="title" />
                <result column="t_context" property="context" />
                <result column="t_note" property="note" />
            </association>
        </collection>
        <discriminator javaType="int" column="sex">
            <case value="1" resultMap="maleHealthFormMapper" />
            <case value="2" resultMap="femaleHealthFormMapper" />
        </discriminator>
    </resultMap>


    <resultMap type="com.xhbjava.domain.MaleEmployee"
        id="maleHealthFormMapper" extends="employee">
        <association property="maleHealthForm" column="id"
            javaType="com.xhbjava.domain.MaleHealthForm">
            <id column="h_id" property="id" />
            <result column="h_heart" property="heart" />
            <result column="h_liver" property="liver" />
            <result column="h_spleen" property="spleen" />
            <result column="h_lung" property="lung" />
            <result column="h_kidney" property="kidney" />
            <result column="h_prostate" property="prostate" />
            <result column="h_note" property="note" />
        </association>
    </resultMap>

    <resultMap type="com.xhbjava.domain.FemaleEmployee"
        id="femaleHealthFormMapper2" extends="employee">
        <association property="femaleHealthForm" column="id"
            javaType="com.xhbjava.domain.FemaleHealthForm">
            <id column="h_id" property="id" />
            <result column="h_heart" property="heart" />
            <result column="h_liver" property="liver" />
            <result column="h_spleen" property="spleen" />
            <result column="h_lung" property="lung" />
            <result column="h_kidney" property="kidney" />
            <result column="h_uterus" property="uterus" />
            <result column="h_note" property="note" />
        </association>
    </resultMap>

    <select id="getEmployee" parameterType="long"
        resultMap="employee">
        select
        emp.id, emp.real_name, emp.sex, emp.birthday,
        emp.mobile, emp.email,
        emp.position, emp.note,
        et.id as et_id,
        et.task_id as et_task_id,
        et.task_name as et_task_name,
        et.note as
        et_note,
        if (emp.sex = 1,
        mhf.id, fhf.id) as h_id,
        if (emp.sex = 1,
        mhf.heart, fhf.heart) as
        h_heart,
        if (emp.sex = 1, mhf.liver, fhf.liver)
        as h_liver,
        if (emp.sex
        = 1, mhf.spleen, fhf.spleen) as h_spleen,
        if
        (emp.sex = 1, mhf.lung,
        fhf.lung) as h_lung,
        if (emp.sex = 1,
        mhf.kidney, fhf.kidney) as
        h_kidney,
        if (emp.sex = 1, mhf.note,
        fhf.note) as h_note,
        mhf.prostate
        as h_prostate, fhf.uterus as h_uterus,
        wc.id wc_id, wc.real_name
        wc_real_name, wc.department wc_department,
        wc.mobile wc_mobile,
        wc.position wc_position, wc.note as wc_note,
        t.id
        as t_id, t.title as
        t_title, t.context as t_context, t.note as t_note
        from t_employee emp
        left join t_employee_task et on emp.id = et.emp_id
        left join
        t_female_health_form fhf on emp.id = fhf.emp_id
        left join
        t_male_health_form mhf on emp.id = mhf.emp_id
        left join t_work_card wc
        on emp.id = wc.emp_id
        left join t_task t on et.task_id = t.id
        where
        emp.id = #{id}
    </select>
</mapper>

2.执行测试类，日志输出

 3.总结

　　通过日志我们可以看出完全消除N+1问题，但是这样做也有弊端。首先，SQL会比较复杂，不好维护。其次，需要配置的比之复杂。所以对于这种方式我们可以针对业务逻辑比较简单的sql进行使用。

二、多对多级联

1.概述

　　在实际生活工作中有多对多级联，我们在程序处理中一般会将多对多级联进行拆分，我们拆分成两个一对多级联就可以。

2.业务场景分析

我们在使用一个系统时涉及到权限管理，我们一个用户可以拥有多个角色，而一个角色又可以对应多个用户，对于这种多对多的情况我们拆分成 两个一对多的关系，一个角色对应多个用户和一个用户对应多个角色。

3.数据库表设计

 4.测试

（1）编写pojo实体类

package com.xhbjava.domain;

import java.util.List;

import com.xhbjava.enumeration.SexEnum;
/**
 * 用户类
 * @author Mr.wang
 *@date    2020年2月28日
 */
public class User {
    private Long id;
    private String userName;
    private String realName;
    private SexEnum sex;
    private String moble;
    private String email;
    private String note;
    //对角色进行一对多关联
    private List<Role> roleList;
    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;
    }
    public String getRealName() {
        return realName;
    }
    public void setRealName(String realName) {
        this.realName = realName;
    }
    public SexEnum getSex() {
        return sex;
    }
    public void setSex(SexEnum sex) {
        this.sex = sex;
    }
    public String getMoble() {
        return moble;
    }
    public void setMoble(String moble) {
        this.moble = moble;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    public String getNote() {
        return note;
    }
    public void setNote(String note) {
        this.note = note;
    }
    public List<Role> getRoleList() {
        return roleList;
    }
    public void setRoleList(List<Role> roleList) {
        this.roleList = roleList;
    }
    

}

package com.xhbjava.domain;

import java.util.List;
/**
 * 角色类
 * @author Mr.wang
 *@date    2020年2月28日
 */
public class Role {
    private Long id;
    private String roleName;
    private String note;
    //关联用户信息，一对多
    private List<User> userList;
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getRoleName() {
        return roleName;
    }
    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }
    public String getNote() {
        return note;
    }
    public void setNote(String note) {
        this.note = note;
    }
    public List<User> getUserList() {
        return userList;
    }
    public void setUserList(List<User> userList) {
        this.userList = userList;
    }
    

}

（2）持久层dao接口编写

package com.xhbjava.dao;

import java.util.List;

import com.xhbjava.domain.Role;

/**
 * 角色接口
 * 
 * @author Mr.wang
 * @date 2020年2月28日
 */
public interface IRoleDao {
    /**
     * 根据id查询角色
     * 
     * @param id
     * @return
     */
    public Role getRole(Long id);

    /**
     * 根据用户id查询角色
     * 
     * @param userId
     * @return
     */
    public List<Role> findRoleByUserId(Long userId);
}

package com.xhbjava.dao;

import java.util.List;

import com.xhbjava.domain.User;

/**
 * 用户接口
 * 
 * @author Mr.wang
 * @date 2020年2月28日
 */
public interface IUserDao {
    /**
     * 根据id获取用户
     * 
     * @param id
     * @return
     */
    public User getUser(Long id);

    /**
     * 根据角色id获取用户
     * 
     * @param roleId
     * @return
     */
    public List<User> findUserByRoleId(Long roleId);
}

（3）持久层映射文件编写

<?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.xhbjava.dao.IUserDao">
    <resultMap type="com.xhbjava.domain.User"
        id="userMapper">
        <id column="id" property="id" />
        <result column="user_name" property="userName" />
        <result column="real_name" property="realName" />
        <result column="sex" property="sex"
            typeHandler="com.xhbjava.typeHandler.SexTypeHandler" />
        <result column="mobile" property="moble" />
        <result column="email" property="email" />
        <result column="position" property="position" />
        <result column="note" property="note" />
        <collection property="roleList" column="id"
            fetchType="lazy"
            select="com.xhbjava.dao.IRoleDao.findRoleByUserId" />
    </resultMap>
    <select id="getUser" parameterType="long" resultMap="userMapper">
        select id,
        user_name, real_name, sex, moble, email, note from t_user where
        id
        =#{id}
    </select>
    <select id="findUserByRoleId" parameterType="long"
        resultMap="userMapper">
        select u.id, u.user_name, u.real_name, u.sex, u.moble,
        u.email, u.note
        from
        t_user u , t_user_role ur where u.id = ur.user_id
        and ur.role_id =#{roleId}
    </select>

</mapper>

<?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.xhbjava.dao.IRoleDao">
    <resultMap type="com.xhbjava.domain.Role"
        id="roleMapper">
        <id column="id" property="id" />
        <result column="role_name" property="roleName" />
        <result column="note" property="note" />
        <collection property="userList" column="id"
            fetchType="lazy"
            select="com.xhbjava.dao.IUserDao.findUserByRoleId" />
    </resultMap>

    <select id="getRole" parameterType="long" resultMap="roleMapper">
        select id,
        role_name, note from t_role where id = #{id}
    </select>

    <select id="findRoleByUserId" parameterType="long"
        resultMap="roleMapper">
        select r.id, r.role_name, r.note from t_role r, t_user_role ur
        where r.id = ur.role_id and ur.user_id = #{userId}
    </select>

</mapper>

（4）编写测试类进行测试

@Test
    public void  testManyToMany() {
        // 5.创建Dao的代理对象
        roleDao = session.getMapper(IRoleDao.class);
        Role role = roleDao.getRole(1L);
        role.getUserList();
        userDao = session.getMapper(IUserDao.class);
        userDao.getUser(1l);
        

    }

 一共有3条sql被执行，调用getUserList获取用户信息所以有第二条sql执行，我们在映射文件中进行了懒加载设置，这样不需要的数据就不会立即加载出来。