3.SpringBoot整合Mybatis(一对多)

前言：

　　Mybatis一对多的处理关系：

一个人有好多本书，每本书的主人只有一个人。当我们查询某个人拥有的所有书籍时，就涉及到了一对多的映射关系。

一、添加数据表：

CREATE TABLE `book` (
  `id` int(6) NOT NULL,
  `name` varchar(50) DEFAULT NULL,
  `uid` int(6) DEFAULT NULL,
  `price` double DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `bu_id` (`uid`),
  CONSTRAINT `bu_id` FOREIGN KEY (`uid`) REFERENCES `user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

 二、代码实现：

1.添加Book实体：

package com.beilin.entity;
/*
   * 书的实体
   * @author 北林
   *
   */

public class Book {

    private int id;
    private int uid;
    private String name;
    private double price;

    public int getId() {
        return id;
    }

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

    public int getUid() {
        return uid;
    }

    public void setUid(int uid) {
        this.uid = uid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getPrice() {
        return price;
    }

    public void setPrice(double price) {
        this.price = price;
    }
}

2.在User实体中添加book集合：

public class User {
    /**
     *  name:学生实体
     */

    //主键id
    private int id;
    //姓名
    private String name;
    //年龄
    private int age;
    //添加book集合
    private List<Book> books;

    // Get和 Set方法
    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public List<Book> getBooks() {
        return books;
    }

    public void setBooks(List<Book> books) {
        this.books = books;
    }
}

3.在UserMapper接口中定义查询方法：

package com.beilin.mapper;

import com.beilin.entity.User;

import java.util.List;

public interface UserMapper {

     //插入
    public void insert(User user);

    //根据id删除
    public void delete(Integer id);

     //根据user的id修改
    public void update(User user);

    //根据id查询
    public User getById(Integer id);

    //查询全部
    public List<User> list();

    /**
          * 根据id查询所有的书
           * @param id
          */
    public User selectBookById(Integer id);


}

4.在mapper映射关系中，添加一对多的select和resaultMap：

注意：当多个表的字段名一样的时候，查询需要用别名

<?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.beilin.mapper.UserMapper">

<!-- 插入一个user -->
<insert id="insert" parameterType="user" useGeneratedKeys="true" keyProperty="id">
        insert into user(name,age) values(#{name},#{age})
    </insert>

<!-- 根据id删除user -->
<delete id="delete" parameterType="int">
        delete  from user where id=#{id}
    </delete>

<!-- 根据id修改user信息 -->
<update id="update" parameterType="user">
        update user set name=#{name},age=#{age} where id=#{id}
    </update>

<!-- 根据id查询 -->
<select id="getById" parameterType="int" resultType="user">
        select * from user where id=#{id}
    </select>

<!-- 查询所有 -->
<select id="list" parameterType="int" resultType="user">
        select * from user
    </select>


<resultMap id="bookMap" type="user">
    <id property="id" column="id"/>
    <result property="name" column="name"/>
    <result property="age" column="age"/>
    <collection property="books" ofType="book">
        <id property="id" column="bid"/>
        <result property="name" column="bookName"/>
        <result property="price" column="price"/>
    </collection>
</resultMap>

<!--根据id查询所有的书  -->
<select id="selectBookById" parameterType="int" resultMap="bookMap">
        select a.*,b.id bid,b.name as "bookName"  ,b.price from user a,book b where a.id=b.uid and a.id=#{id};
    </select>

</mapper>

5.在UserController中实现查询：

package com.beilin.controller;

import com.beilin.entity.Book;
import com.beilin.entity.User;
import com.beilin.mapper.UserMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@RestController
public class UserController {

    @Autowired
    private UserMapper userMapper;

        //插入user
    @RequestMapping("/user")
    public void insert( User user) {
        userMapper.insert(user);
    }

      //根据id删除
    @RequestMapping("/user1/{id}")
    public void delete(@PathVariable("id") Integer id) {
        userMapper.delete(id);
    }
        //修改
    @RequestMapping("/user2/{id}")
    public void update(User user,@PathVariable("id") Integer id) {
        userMapper.update(user);
    }

     //根据id查询user
    @RequestMapping("/user3/{id}")
    public User getById(@PathVariable("id") Integer id) {
        User user = userMapper.getById(id);
        return user;
    }

    //查询全部
    @RequestMapping("/users")
    public List<User> list(){
        List<User> users = userMapper.list();
        return users;
    }

    /**
          * 根据id查询所有的书
           */
    @GetMapping("/user/book/{id}")
    public User getBooks(@PathVariable("id") Integer id){
        User user = userMapper.selectBookById(id);
        return user;
    }
}

三、测试结果：

1.数据库查询结果：

2.postman访问结果：