MyBatis 級聯操作

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關鍵在於練習 resultMap、association、collection 標籤的使用

項目原代碼

一對一 (student -> class)

student 包含個人基本信息和內嵌對象 classes。
在查詢student時，需要聯合 student表和classes表進行查詢。

數據表

CREATE TABLE classes(
    id int not null primary key,
    name varchar(20)
);

insert into classes (id,name) values (1,"一班"),(2,"二班"),(3,"三班");

CREATE TABLE student(
    id int not null primary key auto_increment,
    name varchar(10),
    cid int,
    foreign key (cid) references classes(id)
);

insert into student (id, name, cid) 
    values (1,"KawYang",1),(2,"KawYang",2),(3,"張三",2),(4,"李四",3),(5,"Tom",3),(6,"Jary",3);

實體類

• Classes類對象

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Classes{
    private int id;
    private String name;
}

• Student類對象

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student{
    private int id;
    private String name;
    private Classes classes;
}

Repository

編寫數據庫操作的接口

public interface IStudentRepository {
    /**
     * find by id
     * @param id
     * @return student
     */
    Student findById(int id);

    /**
     * find all
     * @return student list
     */
    List<Student> findAll();
}

Mapper

mapper🏷️ : namespace 屬性映射到 student 的操作接口。
resultMap🏷️ : 配置類對象 --> 在 select 標籤中，使用 resultMap 屬性進行映射。
association🏷️ : 注入內嵌對象，javaType 將內嵌對象映射到實體類。

<?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.kawyang.repository.IStudentRepository">
    
    <resultMap id="student" type="com.kawyang.entity.Student">
        <id property="id" column="sid"/>
        <result property="name" column="sname" />
        <association property="classes" ofType="com.kawyang.entity.Classes">
            <id property="id" column="cid" />
            <result property="name" column="cname" />
        </association>
    </resultMap>

    <select id="findById" resultMap="student" parameterType="int">
        select s.id as sid,s.name as sname, c.id as cid, c.name as cname
        from student s,classes c
        where s.cid=c.id and s.id=#{id}
    </select>

    <select id="findAll" resultMap="student">
        select s.id as sid,s.name as sname, c.id as cid, c.name as cname
        from student s,classes c
        where s.cid=c.id
    </select>
</mapper>

一對多 (class -> studentes)

一個班級含有多個👨‍🎓，通過查詢班級信息，能夠將班級中的所有學生信息查詢出來
需要聯合 classes類和student進行查詢。

數據表

同上

實體類

• ClassesStu

@Data
@AllArgsConstructor
@NoArgsConstructor
public class ClassesStu{
    int id;
    String name;
    List<Student> students;
}

Repository

public interface IClassesStuRepository {
    /**
     * find classesStu by id
     * @param id key
     * @return classesStu
     */
    ClassesStu findById(int id);

    /**
     * find all classesStu
     * @return classesStu list
     */
    List<ClassesStu> findAll();
}

Mapper

配置文件resultMap

    <resultMap id="classesStu" type="com.kawyang.entity.ClassesStu">
        <id property="id" column="cid" />
        <result property="name" column="cname" />
        <collection property="students" ofType="com.kawyang.entity.Student" >
            <id property="id" column="sid" />
            <result property="name" column="sname" />
        </collection>
    </resultMap>

collection🏷️ : 將查詢的 cid & cname 相同的結果，將 sid 和 sname 封裝成 ofType 指定類型的對象，並將多個對象以集合的方式返回。

==>  Preparing: select c.id as cid, c.name as cname, s.id as sid, s.name as sname from student s, classes c where c.id=s.cid
==> Parameters: 
<==    Columns: cid, cname, sid, sname
<==        Row: 1, 一班, 1, KawYang
<==        Row: 2, 二班, 2, KawYang
<==        Row: 2, 二班, 3, 張三
<==        Row: 3, 三班, 4, 李四
<==        Row: 3, 三班, 5, Tom
<==        Row: 3, 三班, 6, Jary
<==      Total: 6

多對多 (goods <-> consumers)

一種商品可以被多個消費者購買
一個消費者可以購買多個商品
商品與消費者事多對多的關係。

數據表

good : 商品信息表
consumer : 消費者信息表
con_good : 中間表

CREATE TABLE good(
    id int primary key not null ,
    name varchar(30)
);

insert into good (id, name) VALUES (1,"電視"),(2, "電冰箱"),(3, "洗衣機"),(4,"筆記本");

CREATE TABLE consumer(
    id int primary key not null,
    name varchar(30)
);

insert into consumer (id, name) VALUES  (1,"KawYang"),(2,"李四"),(3,"張三");

CREATE TABLE con_good(
    id int primary key not null ,
    gid int,
    cid int,
    foreign key (gid) references good(id),
    foreign key (cid) references consumer(id)
);

insert into con_good (id, gid, cid) VALUES (1,1,1),(2,3,2),(3,1,3),(4,2,1),(5,4,2),(6,3,3);

Mapper

多對多相當於兩個一對多的關係，需要配置兩個xml文件

• IGoodMapper

<mapper namespace="com.kawyang.repository.IGoodRepository">
    <resultMap id="good" type="com.kawyang.entity.Good">
        <id property="id" column="gid"/>
        <result property="name" column="gname" />
        <collection property="consumers" ofType="com.kawyang.entity.Consumer">
            <id property="id" column="cid" />
            <result property="name" column="cname" />
        </collection>
    </resultMap>

    <select id="findAll" resultMap="good" >
        select g.id as gid,g.name as gname, c.id as cid, c.name as cname
        from good g,consumer c,con_good m
        where g.id=m.gid and m.cid=c.id
    </select>

    <select id="findById" resultMap="good" parameterType="int">
        select c.id as cid,c.name as cname,g.id as gid,g.name as gname
        from Good as g,Consumer as c, Con_good m
        where c.id = m.cid and g.id=m.gid and g.id=#{id}
    </select>
</mapper>

• IConsumerMapper

<mapper namespace="com.kawyang.repository.IConsumerRepository">
    <resultMap id="consumer" type="com.kawyang.entity.Consumer" >
        <id column="id" property="id" />
        <result column="name" property="name" />
        <collection property="goods" ofType="com.kawyang.entity.Good" >
            <id column="gid" property="id" />
            <result column="gname" property="name" />
        </collection>
    </resultMap>

    <select id="findAll" resultMap="consumer" >
        select c.id as id,c.name as name,g.id as gid,g.name as gname
        from Good as g,Consumer as c, Con_good m
        where c.id = m.cid and g.id=m.gid;
    </select>

    <select id="findById" resultMap="consumer" parameterType="int">
        select c.id as id,c.name as name,g.id as gid,g.name as gname
        from Good as g,Consumer as c, Con_good m
        where c.id = m.cid and g.id=m.gid and g.id=#{id}
    </select>
</mapper>