Mybatis學習筆記（5）-高級映射之多對多映射

時間 2019-11-07

標籤 mybatis 學習筆記高級映射多對欄目 MyBatis 简体版

原文原文鏈接

MyBatis學習筆記（1）—使用篇

MyBatis學習筆記（2）—映射關係篇java

MyBatis學習筆記（3）—高級映射之一對一映射sql

Mybatis學習筆記（4）-高級映射之一對多映射bash

Mybatis學習筆記（5）-高級映射之多對多映射app

假設咱們如今有這樣的需求，想知道某一位顧客購買了哪些商品，可是經過觀察customers表和products表，咱們發現這兩張表並無關聯，那麼怎麼辦呢？經過前面的例子，咱們能夠發現，經由orders和orderitems這兩張表，customers表和products表能創建起間接聯繫。post

咱們以customer表做爲主表，那麼查詢的語句應該是學習

SELECT c.*, o.*, oi.*, p.*
FROM customers AS c, orders AS o, orderitems AS oi, products AS p
WHERE c.cust_id=o.cust_id
AND o.order_num=oi.order_num
AND oi.prod_id=p.prod_id
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根據上一節的解釋，若是咱們使用resultType來定義返回類型，那麼定義的POJO類將會包含不少類似的部分，因此最好仍是採用resultMap來定義返回的類型，這樣咱們就能夠在原有的Customer類上擴展屬性來存儲訂單，那麼訂單屬性內部就會包含訂單項，而訂單項就會跟具體的商品關聯，關聯關係看上面的圖很容易明白。測試

那麼咱們具體來看看類被咱們定義成什麼樣子了。ui

POJO定義

public class Customer {
    private Integer custId;
    private String custName;
    private String custAddress;
    private String custCity;
    private String custState;
    private String custZip;
    private String custCountry;
    private String custContact;
    private String custEmail;
    //新增的orderList用於存儲該客戶所下的訂單
    List<Orders> ordersList;
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Orders類中咱們把原先的Customer屬性刪去，別的都不動spa

Orders中包含OrderItems屬性，OrderItems這麼定義：code

public class OrderItems {
    private Integer orderNum;
    private Integer orderItem;
    private String prodId;
    private Integer quantity;
    private Double itemPrice;
    //新增的商品類，那麼就能夠看到訂單項中包含的商品的具體信息
    private Products products;
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定義resultMap

<resultMap id="CustomerAndProductsMap" type="com.shuqing28.pojo.Customer">
        <id column="cust_id" property="custId"/>
        <result column="cust_name" property="custName"/>
        <result column="cust_address" property="custAddress"/>
        <result column="cust_city" property="custCity"/>
        <result column="cust_state" property="custState"/>
        <result column="cust_zip" property="custZip"/>
        <result column="cust_country" property="custCountry"/>
        <result column="cust_contact" property="custContact"/>
        <result column="cust_email" property="custEmail"/>

        <collection property="ordersList" ofType="com.shuqing28.pojo.Orders">
            <id column="order_num" property="orderNum"/>
            <result column="order_date" property="orderDate"/>
            <result column="cust_id" property="custId"/>
            <collection property="orderItems" ofType="com.shuqing28.pojo.OrderItems">
                <id column="order_num" property="orderNum"/>
                <id column="order_item" property="orderItem"/>
                <result column="prod_id" property="prodId"/>
                <result column="quantity" property="quantity"/>
                <result column="item_price" property="itemPrice"/>
                <association property="products" javaType="com.shuqing28.pojo.Products">
                    <id column="prod_id" property="prodId"/>
                    <result column="vend_id" property="vendId"/>
                    <result column="prod_name" property="prodName"/>
                    <result column="prod_price" property="prodPrice"/>
                    <result column="prod_desc" property="prodDesc"/>
                </association>
            </collection>
        </collection>
    </resultMap>
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咱們從resultMap的定義中就能夠看出從Customer到Products的嵌套關係。下面繼續看查詢語句定義：

<select id="findCustomerProducts" resultMap="CustomerAndProductsMap">
        SELECT c.*, o.*, oi.*, p.*
        FROM customers AS c, orders AS o, orderitems AS oi, products AS p
        WHERE c.cust_id=o.cust_id
        AND o.order_num=oi.order_num
        AND oi.prod_id=p.prod_id
    </select>
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經過一系列內鏈接，查詢出與customer有關聯的products。

定義接口：

public List<Customer> findCustomerProducts();
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測試代碼：

@Test
    public void findCustomersProductsMap(){
        SqlSession sqlSession = sqlSessionFactory.openSession();
        try {
            OrdersDao ordersDao = sqlSession.getMapper(OrdersDao.class);
            List<Customer> customerProducts = ordersDao.findCustomerProducts();
            System.out.println(customerProducts);
        } finally {
            sqlSession.close();
        }
    }
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