MyBatis之多对一查询

接续上一节：MyBatis之一对多查询。

本节的目标是利用Category和Product实现多对一查询，查出来的多个Product属于同一个Category。

修改实体类

为Product类添加Category属性，并为其添加Get和Set方法，如下所示：

package com.nyf.pojo;
 
public class Product {
    private int id;
    private String name;
    private float price;
    private Category category;
    
    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 float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
    public Category getCategory() {
        return category;
    }
    public void setCategory(Category category) {
        this.category = category;
    }
    @Override
    public String toString() {
        return "Product [id=" + id + ", name=" + name + ", price=" + price + "]";
    }
 
}

新增配置文件

在com.nyf.config包下新建 Product.xml 文件，内容如下所示：

<?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.nyf.pojo">
        <resultMap type="Product" id="productBean">
            <id column="pid" property="id" />
            <result column="pname" property="name" />
            <result column="price" property="price" />
     
            <!-- 多对一的关系 -->
            <!-- property: 指的是属性名称, javaType：指的是属性的类型 -->
            <association property="category" javaType="Category">
                <id column="cid" property="id"/>
                <result column="cname" property="name"/>
            </association>
        </resultMap>
     
        <!-- 根据id查询Product, 关联将Orders查询出来 -->
        <select id="listProduct" resultMap="productBean">
            select c.id ‘cid‘, p.id ‘pid‘, c.name ‘cname‘, p.name ‘pname‘ from category_ c left join product_ p on c.id = p.cid
        </select>   
    </mapper>

修改核心配置文件

mybatis-config.xml文件的mappers标签内添加Product.xml映射，如下所示：

<?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>
    <!-- typeAliases用来给com.nyf.pojo.Category取别名为Category -->
    <typeAliases>
      <typeAlias alias="Category" type="com.nyf.pojo.Category" />
      <typeAlias alias="Product" type="com.nyf.pojo.Product" />
    </typeAliases>
    
    <!-- environments配置要连接的数据库 -->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/nyf?characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
            </dataSource>
        </environment>
    </environments>
    
    <!-- mappers,指明实体类对应的配置文件 -->
    <mappers>
        <mapper resource="com/nyf/config/Category.xml"/>
        <mapper resource="com/nyf/config/Product.xml"/>
    </mappers>
</configuration>

编写测试类

新增测试类ManyToOne

package com.nyf.tests;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

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 com.nyf.pojo.Product;
public class ManyToOne {
	public static void main(String[] args) throws IOException {
        String resource = "com/nyf/config/mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
  
        List<Product> ps = session.selectList("listProduct");
        for (Product p : ps) {
            System.out.println(p+" 对应的分类是 \t "+ p.getCategory());
        }
 
        session.commit();
        session.close();
  
    }
}

结果验证

正确结果应该如下所示：

Product [id=1, name=product a, price=0.0] 对应的分类是 	 Category [id=1, name=category1]
Product [id=2, name=product b, price=0.0] 对应的分类是 	 Category [id=1, name=category1]
Product [id=3, name=product c, price=0.0] 对应的分类是 	 Category [id=1, name=category1]
Product [id=4, name=product x, price=0.0] 对应的分类是 	 Category [id=2, name=category2]
Product [id=5, name=product y, price=0.0] 对应的分类是 	 Category [id=2, name=category2]
Product [id=6, name=product z, price=0.0] 对应的分类是 	 Category [id=2, name=category2]

MyBatis之多对一查询

原文：https://www.cnblogs.com/nyfblog/p/13861613.html