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MyBatis初级实战之五:一对一关联查询

其他 程序员欣宸 2584次浏览 0个评论

欢迎访问我的GitHub

https://github.com/zq2599/blog_demos

内容:所有原创文章分类汇总及配套源码,涉及Java、Docker、Kubernetes、DevOPS等;

本篇概览

  • 本文是《MyBatis初级实战》系列的第五篇,从多表获取数据是个常见的场景,一般有以下两种方式:
  • 联表查询:join操作,一次查询完成
  • 多次查询:用第一次查询的结果作为条件,再做查询(MyBatis中叫做嵌套查询)
  • 本篇的内容就是学习MyBatis对上述两种查询的支持,全文由以下章节组成:
  1. 准备数据;
  2. 本次实战的java工程
  3. 最简单的联表(两个表的数据保存在一个实体类的不同字段);
  4. 一对一联表查询(两个表的数据分别保存在不同实体类,假设是A和B,A是B的成员变量)
  5. 一对一嵌套查询(两个表的数据分别保存在不同实体类,假设是A和B,A是B的成员变量)

源码下载

  1. 如果您不想编码,可以在GitHub下载所有源码,地址和链接信息如下表所示(https://github.com/zq2599/blog_demos):
名称 链接 备注
项目主页 https://github.com/zq2599/blog_demos 该项目在GitHub上的主页
git仓库地址(https) https://github.com/zq2599/blog_demos.git 该项目源码的仓库地址,https协议
git仓库地址(ssh) git@github.com:zq2599/blog_demos.git 该项目源码的仓库地址,ssh协议
  1. 这个git项目中有多个文件夹,本章的应用在mybatis文件夹下,如下图红框所示:

MyBatis初级实战之五:一对一关联查询
3. mybatis是个父工程,里面有数个子工程,本篇的源码在relatedoperation子工程中,如下图红框所示:

MyBatis初级实战之五:一对一关联查询

准备数据

  1. 本次实战,在名为mybatis的数据库中建立两个表(和前面几篇文章中的表结构一模一样):user和log表;
  2. user表记录用户信息,非常简单,只有三个字段:主键、名称、年龄
  3. log表记录用户行为,四个字段:主键、用户id、行为描述、行为时间
  4. user和log的关系如下图:

MyBatis初级实战之五:一对一关联查询
5. 建表和添加数据的语句如下:

use mybatis;

DROP TABLE IF EXISTS `user`;

CREATE TABLE `user` (
  `id` int(32) NOT NULL AUTO_INCREMENT,
  `name` varchar(32) NOT NULL,
  `age` int(32) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

DROP TABLE IF EXISTS `log`;

CREATE TABLE `log` (
  `id` int(32) NOT NULL AUTO_INCREMENT,
  `user_id` int(32),
  `action` varchar(255) NOT NULL,
  `create_time` datetime not null,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO mybatis.user (id, name, age) VALUES (3, 'tom', 11);

INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (3, 3, 'read book', '2020-08-07 08:18:16');
INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (4, 3, 'go to the cinema', '2020-09-02 20:00:00');
INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (5, 3, 'have a meal', '2020-10-05 12:03:36');
INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (6, 3, 'have a sleep', '2020-10-06 13:00:12');
INSERT INTO mybatis.log (id, user_id, action, create_time) VALUES (7, 3, 'write', '2020-10-08 09:21:11');

本次实战的java工程

  1. 在父工程mybatis下新建子工程relatedoperation,pom.xml如下:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>com.bolingcavalry</groupId>
        <artifactId>mybatis</artifactId>
        <version>1.0-SNAPSHOT</version>
        <relativePath>../pom.xml</relativePath>
    </parent>

    <groupId>com.bolingcavalry</groupId>
    <artifactId>relatedoperation</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>relatedoperation</name>
    <description>Demo project for Mybatis related operation in Spring Boot</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.mybatis.spring.boot</groupId>
            <artifactId>mybatis-spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-swagger2</artifactId>
        </dependency>
        <!-- swagger-ui -->
        <dependency>
            <groupId>io.springfox</groupId>
            <artifactId>springfox-swagger-ui</artifactId>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid-spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>
  1. 基本配置文件application.yml:
server:
  port: 8080

spring:
  #1.JDBC数据源
  datasource:
    username: root
    password: 123456
    url: jdbc:mysql://192.168.50.43:3306/mybatis?useUnicode=true&characterEncoding=utf-8&useSSL=true&serverTimezone=UTC
    driver-class-name: com.mysql.cj.jdbc.Driver
    #2.连接池配置
    druid:
      #初始化连接池的连接数量 大小,最小,最大
      initial-size: 5
      min-idle: 5
      max-active: 20
      #配置获取连接等待超时的时间
      max-wait: 60000
      #配置间隔多久才进行一次检测,检测需要关闭的空闲连接,单位是毫秒
      time-between-eviction-runs-millis: 60000
      # 配置一个连接在池中最小生存的时间,单位是毫秒
      min-evictable-idle-time-millis: 30000
      # 配置一个连接在池中最大生存的时间,单位是毫秒
      max-evictable-idle-time-millis: 300000
      validation-query: SELECT 1 FROM user
      test-while-idle: true
      test-on-borrow: true
      test-on-return: false
      # 是否缓存preparedStatement,也就是PSCache  官方建议MySQL下建议关闭   个人建议如果想用SQL防火墙 建议打开
      pool-prepared-statements: true
      max-pool-prepared-statement-per-connection-size: 20
      # 配置监控统计拦截的filters,去掉后监控界面sql无法统计,'wall'用于防火墙
      filters: stat,wall,slf4j
      filter:
        stat:
          merge-sql: true
          slow-sql-millis: 5000
      #3.基础监控配置
      web-stat-filter:
        enabled: true
        url-pattern: /*
        #设置不统计哪些URL
        exclusions: "*.js,*.gif,*.jpg,*.png,*.css,*.ico,/druid/*"
        session-stat-enable: true
        session-stat-max-count: 100
      stat-view-servlet:
        enabled: true
        url-pattern: /druid/*
        reset-enable: true
        #设置监控页面的登录名和密码
        login-username: admin
        login-password: admin
        allow: 127.0.0.1
        #deny: 192.168.1.100

# mybatis配置
mybatis:
  # 配置文件所在位置
  config-location: classpath:mybatis-config.xml
  # 映射文件所在位置
  mapper-locations: classpath:mappers/*Mapper.xml

# 日志配置
logging:
  level:
    root: INFO
    com:
      bolingcavalry:
        relatedoperation:
          mapper: debug
  1. 再准备名为application-test.yml的配置文件,这是执行单元测试时用到的,和application.yml的不同之处是spring.datasource.druid.web-stat-filter.enabled配置设置成false
  2. mybatis的配置文件mybatis-config.xml如下:
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <typeAliases>
        <!-- 映射文件中的类不用写全路径了-->
        <package name="com.bolingcavalry.relatedoperation.entity"/>
    </typeAliases>
</configuration>
  1. 数据源配置类DruidConfig.java:
package com.bolingcavalry.relatedoperation;

import com.alibaba.druid.pool.DruidDataSource;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class DruidConfig {

    private static final Logger logger = LoggerFactory.getLogger(DruidConfig.class);

    @Value("${spring.datasource.url}")
    private String dbUrl;
    @Value("${spring.datasource.username}")
    private String username;
    @Value("${spring.datasource.password}")
    private String password;
    @Value("${spring.datasource.driver-class-name}")
    private String driverClassName;
    @Value("${spring.datasource.druid.initial-size}")
    private int initialSize;
    @Value("${spring.datasource.druid.max-active}")
    private int maxActive;
    @Value("${spring.datasource.druid.min-idle}")
    private int minIdle;
    @Value("${spring.datasource.druid.max-wait}")
    private int maxWait;
    @Value("${spring.datasource.druid.pool-prepared-statements}")
    private boolean poolPreparedStatements;
    @Value("${spring.datasource.druid.max-pool-prepared-statement-per-connection-size}")
    private int maxPoolPreparedStatementPerConnectionSize;
    @Value("${spring.datasource.druid.time-between-eviction-runs-millis}")
    private int timeBetweenEvictionRunsMillis;
    @Value("${spring.datasource.druid.min-evictable-idle-time-millis}")
    private int minEvictableIdleTimeMillis;
    @Value("${spring.datasource.druid.max-evictable-idle-time-millis}")
    private int maxEvictableIdleTimeMillis;
    @Value("${spring.datasource.druid.validation-query}")
    private String validationQuery;
    @Value("${spring.datasource.druid.test-while-idle}")
    private boolean testWhileIdle;
    @Value("${spring.datasource.druid.test-on-borrow}")
    private boolean testOnBorrow;
    @Value("${spring.datasource.druid.test-on-return}")
    private boolean testOnReturn;
    @Value("${spring.datasource.druid.filters}")
    private String filters;
    @Value("{spring.datasource.druid.connection-properties}")
    private String connectionProperties;

    /**
     * Druid 连接池配置
     */
    @Bean
    public DruidDataSource dataSource() {
        DruidDataSource datasource = new DruidDataSource();
        datasource.setUrl(dbUrl);
        datasource.setUsername(username);
        datasource.setPassword(password);
        datasource.setDriverClassName(driverClassName);
        datasource.setInitialSize(initialSize);
        datasource.setMinIdle(minIdle);
        datasource.setMaxActive(maxActive);
        datasource.setMaxWait(maxWait);
        datasource.setTimeBetweenEvictionRunsMillis(timeBetweenEvictionRunsMillis);
        datasource.setMinEvictableIdleTimeMillis(minEvictableIdleTimeMillis);
        datasource.setMaxEvictableIdleTimeMillis(minEvictableIdleTimeMillis);
        datasource.setValidationQuery(validationQuery);
        datasource.setTestWhileIdle(testWhileIdle);
        datasource.setTestOnBorrow(testOnBorrow);
        datasource.setTestOnReturn(testOnReturn);
        datasource.setPoolPreparedStatements(poolPreparedStatements);
        datasource.setMaxPoolPreparedStatementPerConnectionSize(maxPoolPreparedStatementPerConnectionSize);
        try {
            datasource.setFilters(filters);
        } catch (Exception e) {
            logger.error("druid configuration initialization filter", e);
        }
        datasource.setConnectionProperties(connectionProperties);
        return datasource;
    }

}
  1. swagger配置类:
package com.bolingcavalry.relatedoperation;

import springfox.documentation.service.Contact;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import springfox.documentation.builders.ApiInfoBuilder;
import springfox.documentation.builders.PathSelectors;
import springfox.documentation.builders.RequestHandlerSelectors;
import springfox.documentation.service.ApiInfo;
import springfox.documentation.service.Tag;
import springfox.documentation.spi.DocumentationType;
import springfox.documentation.spring.web.plugins.Docket;
import springfox.documentation.swagger2.annotations.EnableSwagger2;

@Configuration
@EnableSwagger2
public class SwaggerConfig {

    @Bean
    public Docket createRestApi() {
        return new Docket(DocumentationType.SWAGGER_2)
                .apiInfo(apiInfo())
                .tags(new Tag("UserController", "用户服务"), new Tag("LogController", "日志服务"))
                .select()
                // 当前包路径
                .apis(RequestHandlerSelectors.basePackage("com.bolingcavalry.relatedoperation.controller"))
                .paths(PathSelectors.any())
                .build();
    }

    //构建 api文档的详细信息函数,注意这里的注解引用的是哪个
    private ApiInfo apiInfo() {
        return new ApiInfoBuilder()
                //页面标题
                .title("MyBatis CURD操作")
                //创建人
                .contact(new Contact("程序员欣宸", "https://github.com/zq2599/blog_demos", "zq2599@gmail.com"))
                //版本号
                .version("1.0")
                //描述
                .description("API 描述")
                .build();
    }
}
  1. springboot引导类:
package com.bolingcavalry.relatedoperation;

import org.mybatis.spring.annotation.MapperScan;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
@MapperScan("com.bolingcavalry.relatedoperation.mapper")
public class RelatedOperationApplication {

    public static void main(String[] args) {
        SpringApplication.run(RelatedOperationApplication.class, args);
    }

}
  1. 用户表的实体类:
package com.bolingcavalry.relatedoperation.entity;

import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@ApiModel(description = "用户实体类")
public class User {

    @ApiModelProperty(value = "用户ID")
    private Integer id;

    @ApiModelProperty(value = "用户名", required = true)
    private String name;

    @ApiModelProperty(value = "用户地址", required = false)
    private Integer age;
}
  1. 日志表的实体类:
package com.bolingcavalry.relatedoperation.entity;

import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.sql.Date;

@Data
@NoArgsConstructor
@ApiModel(description = "日志实体类")
public class Log {
    @ApiModelProperty(value = "日志ID")
    private Integer id;

    @ApiModelProperty(value = "用户ID")
    private Integer userId;

    @ApiModelProperty(value = "日志内容")
    private String action;

    @ApiModelProperty(value = "创建时间")
    private Date createTime;
}
  • 以上就是本篇的准备代码,接下来在此基础上实现各种多表关联查询

最简单的联表

  • 先实战的是最普通的联表,如下图所示,查询结果是名为LogExtend的实体类,这个类有5个字段,其中四个来自日志表log,一个来自用户表user:

MyBatis初级实战之五:一对一关联查询

  • 下图是开发步骤:

MyBatis初级实战之五:一对一关联查询

  1. 实体类LogExtend的源码如下,可见和Log相比多了个userName字段:
package com.bolingcavalry.relatedoperation.entity;

import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@ApiModel(description = "日志实体类(含用户表的字段)")
public class LogExtend extends Log {

    @ApiModelProperty(value = "用户名")
    private String userName;
}
  1. 新建log表对应的映射文件LogMapper.xml,如下所示,里面是通过left join语法执行的简单的联表查询,以及查询结果对应的resultMap定义:
<?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.bolingcavalry.relatedoperation.mapper.LogMapper">

    <!--联表查询,返回log对象,该对象有个userName字段,值是user表的user_name字段-->
    <select id="oneObjectSel" parameterType="int" resultMap="logExtendResultMap">
        select l.id as id,
               l.user_id as user_id,
               l.action as action,
               l.create_time as create_time,
               u.name as user_name
        from log as l
                 left join user as u
                           on l.user_id = u.id
        where l.id = #{id}
    </select>

    <resultMap id="logExtendResultMap" type="logExtend">
        <id property="id" column="id"/>
        <result column="user_id" jdbcType="INTEGER" property="userId"/>
        <result column="action" jdbcType="VARCHAR" property="action"/>
        <result column="create_time" jdbcType="TIMESTAMP" property="createTime"/>
        <result column="user_name" jdbcType="VARCHAR" property="userName"/>
    </resultMap>
</mapper>
  1. mapper接口代码:
package com.bolingcavalry.relatedoperation.mapper;

import com.bolingcavalry.relatedoperation.entity.LogAssociateUser;
import com.bolingcavalry.relatedoperation.entity.LogExtend;
import org.springframework.stereotype.Repository;

@Repository
public interface LogMapper {

    LogExtend oneObjectSel(int id);
}
  1. service层的代码在LogService.java文件中:
package com.bolingcavalry.relatedoperation.service;

import com.bolingcavalry.relatedoperation.entity.LogAssociateUser;
import com.bolingcavalry.relatedoperation.entity.LogExtend;
import com.bolingcavalry.relatedoperation.mapper.LogMapper;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class LogService {
    @Autowired
    LogMapper logMapper;

    public LogExtend oneObjectSel(int id){
        return logMapper.oneObjectSel(id);
    }
}
  1. controller层的代码在LogController.java文件中:
@RestController
@RequestMapping("/log")
@Api(tags = {"LogController"})
public class LogController {
    @Autowired
    private LogService logService;

    @ApiOperation(value = "根据ID查找日志记录,带userName字段,该字段通过联表查询实现", notes="根据ID查找日志记录,带userName字段,该字段通过联表查询实现")
    @ApiImplicitParam(name = "id", value = "日志ID", paramType = "path", required = true, dataType = "Integer")
    @RequestMapping(value = "/aggregate/{id}", method = RequestMethod.GET)
    public LogExtend oneObjectSel(@PathVariable int id){
        return logService.oneObjectSel(id);
    }
  1. 编写单元测试的代码ControllerTest.java,由于今天的测试涉及到user和log两个表,因此在测试类ControllerTest的内部准备了两个内部类,分别用于测试user和log表:
package com.bolingcavalry.relatedoperation.controller;

import lombok.extern.slf4j.Slf4j;
import org.junit.jupiter.api.*;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.MediaType;
import org.springframework.test.context.ActiveProfiles;
import org.springframework.test.web.servlet.MockMvc;
import org.springframework.test.web.servlet.request.MockMvcRequestBuilders;

import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

@SpringBootTest
@DisplayName("Web接口的单元测试")
@AutoConfigureMockMvc
@ActiveProfiles("test")
@Slf4j
public class ControllerTest {
    /**
     * 查询方式:联表
     */
    final static String SEARCH_TYPE_LEFT_JOIN = "leftjoin";

    /**
     * 查询方式:嵌套
     */
    final static String SEARCH_TYPE_NESTED = "nested";

    final static int TEST_USER_ID = 3;

    final static String TEST_USER_NAME = "tom";

    @Autowired MockMvc mvc;

    @Nested
    @TestMethodOrder(MethodOrderer.OrderAnnotation.class)
    @DisplayName("用户服务")
    class User {
    
    }
    
    @Nested
    @TestMethodOrder(MethodOrderer.OrderAnnotation.class)
    @DisplayName("日志服务")
    class Log {
        final static int TEST_LOG_ID = 5;

        @Test
        @DisplayName("通过日志ID获取日志信息,带userName字段,该字段通过联表查询实现")
        @Order(1)
        void oneObjectSel() throws Exception {
            mvc.perform(MockMvcRequestBuilders.get("/log/aggregate/" + TEST_LOG_ID)
                    .accept(MediaType.APPLICATION_JSON))
                    .andExpect(status().isOk())
                    .andExpect(jsonPath("$.id").value(TEST_LOG_ID))
                    .andExpect(jsonPath("$.userName").value(TEST_USER_NAME))
                    .andDo(print());
        }
    }
}
  1. 执行上述单元测试方法,结果如下图,红框中就是controller层返回的数据,可见已通过Mybatis成功取得LogExtend实例:

MyBatis初级实战之五:一对一关联查询

  • 下一站是一对一联表查询;

关于一对一关联的两种方式

  • 前面的查询有个特点:尽管查询了两个表,但结果都在同一实体类的不同字段,而更符合业务逻辑的关系应该是log类中有个user类的成员变量,即如下形式:
@Data
@NoArgsConstructor
@ApiModel(description = "日志实体类")
public class LogAssociateUser {
    @ApiModelProperty(value = "日志ID")
    private Integer id;

    @ApiModelProperty(value = "用户对象")
    private User user;

    @ApiModelProperty(value = "日志内容")
    private String action;

    @ApiModelProperty(value = "创建时间")
    private Date createTime;
}
  • 接下来的实战就是如何用MyBatis查询得到上述LogAssociateUser 类型的结果;
  • 一对一关联的实现有联表嵌套查询两种,它们的差异在Mybatis中体现在association的子节点上:
  1. 联表时,association内使用result子节点,将联表查询的结果映射到关联对象;
  2. 嵌套时,association内使用select子节点,触发一次新的查询;

一对一(联表)

所谓一对一,就是一个对象关联了另一个对象,例如一条log记录中,带有对应的user信息;

  1. 下面是新的实体类LogAssociateUser,该类对应的是log表记录,有个user字段,类型是User对象:
@Data
@NoArgsConstructor
@ApiModel(description = "日志实体类")
public class LogAssociateUser {
    @ApiModelProperty(value = "日志ID")
    private Integer id;

    @ApiModelProperty(value = "用户对象")
    private User user;

    @ApiModelProperty(value = "日志内容")
    private String action;

    @ApiModelProperty(value = "创建时间")
    private Date createTime;
}
  1. 映射文件LogMapper.xml中,sql和resultMap如下,可见查询的时候将user表的字段都查出来了,然后在resultMap中用association节点去处理sql中查出的user表的数据,通过javaType属性转为User类的实例:
    <!--联表查询,返回log对象,它的成员变量中有user对象-->
    <select id="leftJoinSel" parameterType="int" resultMap="leftJoinResultMap">
        select l.id as log_id,
               l.action as log_action,
               l.create_time as log_create_time,
               u.id as user_id,
               u.name as user_name,
               u.age as user_age
        from log as l
               left join user as u
               on l.user_id = u.id
        where l.id = #{id}
    </select>

    <resultMap id="leftJoinResultMap" type="LogAssociateUser">
        <id property="id" column="log_id"/>

        <result  property="action" column="log_action" jdbcType="VARCHAR"/>

        <result property="createTime" column="log_create_time" jdbcType="TIMESTAMP" />

        <association property="user" javaType="User">
            <id property="id" column="user_id"/>
            <result property="name" column="user_name"/>
            <result property="age" column="user_age"/>
        </association>
    </resultMap>
  1. 以上就是一对一(联表)的关键点,接下来按部就班的在LogMapper、LogService、LogController中添加方法即可,下面是LogController中对应的web接口,稍后会在单元测试中调用这个接口进行验证:
    @ApiOperation(value = "根据ID查找日志记录,带用户对象,联表查询实现", notes="根据ID查找日志记录,带用户对象,联表查询实现")
    @ApiImplicitParam(name = "id", value = "日志ID", paramType = "path", required = true, dataType = "Integer")
    @RequestMapping(value = "/leftjoin/{id}", method = RequestMethod.GET)
    public LogAssociateUser leftJoinSel(@PathVariable int id){
        return logService.leftJoinSel(id);
    }
  1. 最后是单元测试的代码(ControllerTest.java文件),用来测试上述代码是否有效,注意下面的queryAndCheck私有方法,该方法中发起请求并验证结果:
        /**
         * 通过日志ID获取日志信息有两种方式:联表和嵌套查询,
         * 从客户端来看,仅一部分path不同,因此将请求和检查封装到一个通用方法中,
         * 调用方法只需要指定不同的那一段path
         * @param subPath
         * @throws Exception
         */
        private void queryAndCheck(String subPath) throws Exception {
            String queryPath = "/log/" + subPath + "/" + TEST_LOG_ID;

            log.info("query path [{}]", queryPath);

            mvc.perform(MockMvcRequestBuilders.get(queryPath)
                    .accept(MediaType.APPLICATION_JSON))
                    .andExpect(status().isOk())
                    .andExpect(jsonPath("$.id").value(TEST_LOG_ID))
                    .andExpect(jsonPath("$.user.id").value(TEST_USER_ID))
                    .andDo(print());
        }

        @Test
        @DisplayName("通过日志ID获取日志信息(关联了用户),联表查询")
        @Order(2)
        void leftJoinSel() throws Exception {
            queryAndCheck(SEARCH_TYPE_LEFT_JOIN);
        }
  1. 执行单元测试结果如下,可见:内部嵌套了一个json对象,就是user表的数据:

MyBatis初级实战之五:一对一关联查询

一对一(嵌套)

  1. 接下来试试嵌套的方式;
  2. LogMapper.xml中对应的sql:
    <!--嵌套-->
    <select id="nestedSel" parameterType="int" resultMap="nestedResultMap">
        select
            l.id as log_id,
            l.user_id as log_user_id,
            l.action as log_action,
            l.create_time as log_create_time
        from mybatis.log as l
        where l.id = #{id}
    </select>
  1. 上述sql对应的resultMap如下,可见association节点中有个select属性,这就是MyBatis支持嵌套查询的关键,该属性的值是个select节点:
    <!-- association节点的select属性会触发嵌套查询-->
    <resultMap id="nestedResultMap" type="LogAssociateUser">
        <!-- column属性中的log_id,来自前面查询时的"l.id as log_id" -->
        <id property="id" column="log_id"/>
        <!-- column属性中的log_action,来自前面查询时的"l.action as log_action" -->
        <result  property="action" column="log_action" jdbcType="VARCHAR"/>
        <!-- column属性中的log_create_time,来自前面查询时的"l.create_time as log_create_time" -->
        <result property="createTime" column="log_create_time" jdbcType="TIMESTAMP" />
        <!-- select属性,表示这里要执行嵌套查询,将log_user_id传给嵌套的查询 -->
        <association property="user" column="log_user_id" select="selectUserByUserId"></association>
    </resultMap>
  1. 上述节点中select属性的值,对应一个select节点,如下:
    <select id="selectUserByUserId" parameterType="int" resultType="User">
        select
            u.id,
            u.name,
            u.age
        from mybatis.user as u
        where u.id = #{log_user_id}
    </select>
  1. 以上就是一对一(嵌套)的关键点,接下来按部就班的在LogMapper、LogService、LogController中添加方法即可,下面是LogController中对应的web接口,稍后会在单元测试中调用这个接口进行验证:
    @ApiOperation(value = "根据ID查找日志记录,带用户对象,嵌套查询实现", notes="根据ID查找日志记录,带用户对象,嵌套查询实现")
    @ApiImplicitParam(name = "id", value = "日志ID", paramType = "path", required = true, dataType = "Integer")
    @RequestMapping(value = "/nested/{id}", method = RequestMethod.GET)
    public LogAssociateUser nestedSel(@PathVariable int id){
        return logService.nestedSel(id);
    }
  1. 最后是单元测试的代码(ControllerTest.java文件),用来测试上述代码是否有效,如下可见,直接调用了前面的queryAndCheck来验证:
        @Test
        @DisplayName("通过日志ID获取日志信息(关联了用户),嵌套查询")
        @Order(3)
        void nestedSel() throws Exception {
            queryAndCheck(SEARCH_TYPE_NESTED);
        }
  1. 执行上述单元测试代码,结果如下,可见嵌套查询的方式也能将user表的数据成功获取,放入log实例的成员变量中:

MyBatis初级实战之五:一对一关联查询
8. 最后是对比联表和嵌套查询的差异,先看联表查询的MyBatis日志,如下图红框所示,只有一次sql查询:

MyBatis初级实战之五:一对一关联查询
9. 再看嵌套查询的日志,如下图,红框是第一次查询,结果中的userid作为绿框中的第二次查询的条件:

MyBatis初级实战之五:一对一关联查询

  • 至此,一对一的多表查询实战就完成了,本篇的逻辑是一条log记录关联一条user记录,下一篇文章,咱们学习一对多关联,即一个user有多条log记录;

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