如果使用association和collection实现层级查询

1.我们希望返回的实体对象结构如下：

@Data
public class OrderPaymentInfoRetDTO {

    private Integer id;//id字段必须要写，他作为过层级1对多的重要滤条件存在的

    RespOrderListDTO  respOrderListDTO;//一个对象

    List<QueryOrderPaymentDTO>   respOrderAndSubListDtos;//list

}

2.我们定义了两个 resultMap 分别上面的两个类属性

2.1 此resultMap对应 RespOrderListDTO 类

<resultMap id="forgeOrderBaseMap" type="com.zhangmen.fms.trn.forge.model.resp.RespOrderListDTO">
        <id column="id" property="id" jdbcType="INTEGER"/>
        <result column="stu_id" property="stuId" jdbcType="INTEGER"/>
        <result column="sel_id" property="selId" jdbcType="INTEGER"/>
        <result column="paid_sel_id" property="paidSelId" jdbcType="INTEGER"/>
        <result column="owner_sel_id" property="ownerSelId" jdbcType="INTEGER"/>
        <result column="money" property="money" jdbcType="INTEGER"/>
        <result column="amount" property="amount" jdbcType="DECIMAL"/>
        <result column="mins" property="mins" jdbcType="INTEGER"/>
        <result column="gift_mins" property="giftMins" jdbcType="INTEGER"/>
        <result column="is_paid" property="isPaid" jdbcType="INTEGER"/>
        <result column="is_canceled" property="isCanceled" jdbcType="BIT"/>
        <result column="state" property="state" jdbcType="BIT"/>
        <result column="out_trade_no" property="outTradeNo" jdbcType="VARCHAR"/>
        <result column="hours" property="hours" jdbcType="INTEGER"/>
        <result column="meta" property="meta" jdbcType="INTEGER"/>
        <result column="is_split" property="isSplit" jdbcType="BIT"/>
        <result column="deal_time" property="dealTime" jdbcType="TIMESTAMP"/>
        <result column="expired_at" property="expiredAt" jdbcType="TIMESTAMP"/>
        <result column="pay_way" property="payWay" jdbcType="VARCHAR"/>
        <result column="hour_pkg_type" property="hourPkgType" jdbcType="INTEGER"/>
        <result column="created_at" property="createdAt" jdbcType="TIMESTAMP"/>
        <result column="updated_at" property="updatedAt" jdbcType="TIMESTAMP"/>
        <result column="hour_pkg_id" property="hourPkgId" jdbcType="INTEGER"/>
        <result column="total_money"  property="totalMoney" jdbcType="DECIMAL"/>
        <result column="payment_type" property="paymentType" jdbcType="INTEGER"/>
        <result column="refer_id" property="referId" jdbcType="INTEGER"/>
        <result column="hour_pkg_catg" property="hourPkgCatg" jdbcType="BIT"/>
        <result column="stu_grade" property="stuGrade" jdbcType="VARCHAR"/>
        <result column="supply_payment_id" property="supplyPaymentId" jdbcType="INTEGER"/>
        <result column="biz_source" jdbcType="INTEGER" property="bizSource"/>
        <result column="bu" property="ghpBu" jdbcType="INTEGER"/>
    </resultMap>

2.2 此resultMap对应 QueryOrderPaymentDTO 类

<resultMap id="forgeOrderPaymentBaseMap" type="com.zhangmen.fms.trn.forge.model.QueryOrderPaymentDTO">
        <id column="subId" property="orderPaymentId" jdbcType="INTEGER"/>
        <result column="orderId" property="orderId" jdbcType="INTEGER"/>
        <result column="orderNo" property="orderNo" jdbcType="VARCHAR"/>
        <result column="subOutTradeNo" property="orderPaymentNo" jdbcType="VARCHAR"/>
        <result column="subAmount" property="orderPaymentAmount" jdbcType="DECIMAL"/>
        <result column="subIsPaid" property="isPaid" jdbcType="INTEGER"/>
        <result column="subIsCanceled" property="isCanceled" jdbcType="INTEGER"/>
        <result column="subState" property="orderPaymentStatus" jdbcType="INTEGER"/>
        <result column="subDealTime" property="dealTime" jdbcType="TIMESTAMP"/>
        <result column="subPassTime" property="passTime" jdbcType="TIMESTAMP"/>
        <result column="subPayWay" property="payWay" jdbcType="VARCHAR"/>
        <result column="subAuditState" property="auditState" jdbcType="INTEGER"/>
        <result column="subPayType" property="payType" jdbcType="INTEGER"/>
    </resultMap>

3.然后定义 映射到 我们要返回的 OrderPaymentInfoRetDTO类上的实体resultMap

 <resultMap id="orderPaymentInfoRetMap" type="com.zhangmen.fms.trn.forge.model.resp.OrderPaymentInfoRetDTO">
        <id column="id" property="id" jdbcType="INTEGER"/> //作为collection属性的分组过滤条件
        <association property="respOrderListDTO" resultMap="forgeOrderBaseMap" />
        <collection property="respOrderAndSubListDtos" resultMap="forgeOrderPaymentBaseMap"/>
    </resultMap>

4.定义我们的查询SQL

<select id="selectSubPaymentAttachmentPaymentsByCond" parameterType="com.zhangmen.fms.trn.forge.model.req.QueryOrderPaymentReq"
            resultMap="orderPaymentInfoRetMap" >
        SELECT
        p.id,
        p.stu_id,
        p.sel_id,
        p.paid_sel_id,
        p.owner_sel_id,
        p.money,
        p.amount,
        p.mins,
        p.gift_mins,
        p.is_paid,
        p.is_canceled,
        p.state AS state,
        p.out_trade_no,
        p.hours,
        p.meta,
        p.is_split,
        p.deal_time,
        p.expired_at,
        p.pay_way,
        p.hour_pkg_type,
        p.created_at,
        p.updated_at,
        p.hour_pkg_id,
        p.total_money,
        p.payment_type,
        p.refer_id,
        p.hour_pkg_catg,
        p.stu_grade,
        p.supply_payment_id,
        p.biz_source,
        ghp.bu,
        p.id AS orderId,
        p.out_trade_no AS orderNo,
        s.id AS subId,
        s.out_trade_no AS subOutTradeNo,
        s.amount AS subAmount,
        s.is_paid AS subIsPaid,
        s.is_canceled AS subIsCanceled,
        s.state AS subState,
        s.deal_time  AS subDealTime,
        s.pass_time AS subPassTime,
        s.pay_way AS subPayWay,
        s.audit_state AS subAuditState,
        s.pay_type AS subPayType
        from payments p
        INNER JOIN sub_payments s on s.parent_pay_id = p.id
        INNER JOIN grades_hours_prices ghp on ghp.id = p.hour_pkg_id
        WHERE p.is_paid = 1 AND p.is_canceled = 0
        AND s.is_paid = 1 AND s.is_canceled = 0
        <if test="orderNoList != null and orderNoList.size() > 0">
            AND p.out_trade_no IN
            <foreach item="item" index="index" collection="orderNoList" open="(" separator="," close=")">
                #{item}
            </foreach>
        </if>
        <if test="stuId != null">
            AND p.stu_id > #{stuId}
        </if>
        <if test="orderPaymentAmount != null">
            AND s.amount > #{orderPaymentAmount}
        </if>
        ORDER BY p.id DESC, s.deal_time DESC
    </select>

注意：我们查看SQL可知，这样的语句查询出的结果是笛卡尔积并非是层级结构，而我们想要获取到层级结构就需要MyBatis对返回的结果进行封装，将相同的父属性进行合并，然后把子对象附属在其下。

实现此过滤的关键因素就是上面加粗的大字号字体， 注意resultMap中放大的字体，父对象中的 <id column 属性对应的ID列一定要是过滤的主键，同时作为层级结构中的子列表的ID属性也要是其主键，否则无法进行层级关系的转换