九，Mybatis的ManyToOne对象关联查询

商品和详情对象关联查询：多对一

通过n(多)的一方找到1的一方

1. 在n的一方(goodsDetail)的实体类中用“1”的一方(Goods) 创建属性

public class GoodsDetail {
    private int gdId;
    private int goodsId;
    private String gdPicUrl;
    private String gdOrder;
    private Goods goods;

    public Goods getGoods() {
        return goods;
    }

    public void setGoods(Goods goods) {
        this.goods = goods;
    }

    public int getGdId() {
        return gdId;
    }

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

    public int getGoodsId() {
        return goodsId;
    }

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

    public String getGdPicUrl() {
        return gdPicUrl;
    }

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

    public String getGdOrder() {
        return gdOrder;
    }

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

// 创建实体类
public class Goods {
    private int goodsId;
    private String title;
    private String subTitle;
    private float originalCost;
    private float currentPrice;
    private float discount;
    private int isFreeDelivery;
    private int categoryId;
    private List<GoodsDetail> goodsDetails;

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

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

    public int getGoodsId() {
        return goodsId;
    }

    public void setGoodsId(int 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 int getIsFreeDelivery() {
        return isFreeDelivery;
    }

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

    public int getCategoryId() {
        return categoryId;
    }

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

2. 进行对象关联的描述

<?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">
    <resultMap id="rmGoodsDetail" type="com.imooc.mybatis.entity.GoodsDetail">
        <id column="gd_id" property="gdId"></id>
         <!--因为原有的goodsId会作为参数传给联动的SQL，所以这里需要手动为GoodsDetail 的goodsId属性赋值。-->
        <result column="goods_id" property="goodsId"/>
        <association property="goods" select="goods.selectById" column="goods_id"/>
    </resultMap>
    <select id="selectManyToOne" resultMap="rmGoodsDetail">
        select * from t_goods_detail limit 0,15;
    </select>
</mapper>

<?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="goods">
    <select id="selectById" parameterType="int" resultType="com.imooc.mybatis.entity.Goods">
        select * from t_goods where goods_id=#{value}
    </select>
        

3. 测试

   @Test
    public void testManyToOne(){
        SqlSession sqlSession = null;
        try {
            sqlSession = MyBatisUtils.openSession();
            List<GoodsDetail> goodsDetails = sqlSession.selectList("goodsDetail.selectManyToOne");
            for (GoodsDetail goodsDetail:goodsDetails){
                System.out.println(goodsDetail.getGdId()+goodsDetail.getGoods().getTitle());
            }
        } catch (Exception e) {
            throw e;
        } finally {
            MyBatisUtils.closeSqlSession(sqlSession);
        }
    }