022.OneToMany对象关联查询（多表级联查询）

1.案例分析

1.班级（1）---> 学生 （n）  学生必须持有班级的外键
2.1对1  通过主键关联
3.多对多必须单独抽象出一张中间表

 2.商品和详情对象关联查询（一对多）

2.1 创建商品描述详情的实体类

package com.imooc.mybatis.entity;

public class GoodsDetail {
    private Integer gdId;
    private Integer goodsId;
    private String gdPicUrl;
    private Integer gdOrder;


    public Integer getGdId() {
        return gdId;
    }

    public void setGdId(Integer gdId) {
        this.gdId = gdId;
    }

    public Integer getGoodsId() {
        return goodsId;
    }

    public void setGoodsId(Integer goodsId) {
        this.goodsId = goodsId;
    }

    public String getGdPicUrl() {
        return gdPicUrl;
    }

    public void setGdPicUrl(String gdPicUrl) {
        this.gdPicUrl = gdPicUrl;
    }

    public Integer getGdOrder() {
        return gdOrder;
    }

    public void setGdOrder(Integer gdOrder) {
        this.gdOrder = gdOrder;
    }

}

2.2 增加goodsDetail对应的mapper文件（对象关联）

<?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="goodsDetail">

    <select id="selectByGoodsId" parameterType="Integer"
            resultType="com.imooc.mybatis.entity.GoodsDetail">
        select *
        from t_goods_detail
        where goods_id = #{value}
    </select>

</mapper>

2.3  一的一方如何持有多的一方    private List<GoodsDetail> goodsDetails;

package com.imooc.mybatis.entity;

import java.util.List;

public class Goods
{
    private Integer goodsId;//商品编号
    private String title;//标题
    private String subTitle;//子标题
    private Float originalCost;//原始价格
    private Float currentPrice;//当前价格
    private Float discount;//折扣率
    private Integer isFreeDelivery;//是否包邮 ,1-包邮 0-不包邮
    private Integer categoryId;//分类编号
    private List<GoodsDetail> goodsDetails;

    public List<GoodsDetail> getGoodsDetails()
    {
        return goodsDetails;
    }

    public void setGoodsDetails(List<GoodsDetail> goodsDetails)
    {
        this.goodsDetails = goodsDetails;
    }

    public Integer getGoodsId()
    {
        return goodsId;
    }

    public void setGoodsId(Integer goodsId)
    {
        this.goodsId = goodsId;
    }

    public String getTitle()
    {
        return title;
    }

    public void setTitle(String title)
    {
        this.title = title;
    }

    public String getSubTitle()
    {
        return subTitle;
    }

    public void setSubTitle(String subTitle)
    {
        this.subTitle = subTitle;
    }

    public Float getOriginalCost()
    {
        return originalCost;
    }

    public void setOriginalCost(Float originalCost)
    {
        this.originalCost = originalCost;
    }

    public Float getCurrentPrice()
    {
        return currentPrice;
    }

    public void setCurrentPrice(Float currentPrice)
    {
        this.currentPrice = currentPrice;
    }

    public Float getDiscount()
    {
        return discount;
    }

    public void setDiscount(Float discount)
    {
        this.discount = discount;
    }

    public Integer getIsFreeDelivery()
    {
        return isFreeDelivery;
    }

    public void setIsFreeDelivery(Integer isFreeDelivery)
    {
        this.isFreeDelivery = isFreeDelivery;
    }

    public Integer getCategoryId()
    {
        return categoryId;
    }

    public void setCategoryId(Integer categoryId)
    {
        this.categoryId = categoryId;
    }

}

2.4  goods.xml (对2.3中添加的list数据进行说明)

<!--
         resultMap可用于说明一对多或者多对一的映射逻辑
         id 是resultMap属性引用的标志
         type 指向One的实体(Goods)
     -->
    <resultMap id="rmGoods1" type="com.imooc.mybatis.entity.Goods">
        <!-- 映射goods对象的主键到goods_id字段 -->
        <id column="goods_id" property="goodsId"/>
        <!--
            collection的含义是,在
            select * from t_goods limit 0,1 得到结果后,对所有Goods对象遍历得到goods_id字段值,
            并代入到goodsDetail命名空间的findByGoodsId的SQL中执行查询,
            将得到的"商品详情"集合赋值给goodsDetails List对象.
        -->
        <collection property="goodsDetails" select="goodsDetail.selectByGoodsId"
                    column="goods_id"/>
    </resultMap>

    <select id="selectOneToMany" resultMap="rmGoods1">
        select * from t_goods limit 0,10
    </select>

2.5 核心配置文件中进行注册mybatis-config.xml

<!--声明mapper的配置文件-->
    <mappers>
        <mapper resource="mappers/goods_detail.xml"/>
    </mappers>

2.6 测试用例

/**
     * 一对多对象关联查询
     * @throws Exception
     */
    @Test
    public void testOneToMany() throws Exception {
        SqlSession session = null;
        try {
            session = MyBatisUtils.openSession();
            List<Goods> list = session.selectList("goods.selectOneToMany");
            for(Goods goods:list) {
                System.out.println(goods.getTitle() + ":" + goods.getGoodsDetails().size());
            }
        } catch (Exception e) {
            throw e;
        } finally {
            MyBatisUtils.closeSession(session);
        }
    }