Mybatis学习笔记（5）-高级映射之多对多映射

MyBatis学习笔记（1）—使用篇

MyBatis学习笔记（2）—映射关系篇

MyBatis学习笔记（3）—高级映射之一对一映射

Mybatis学习笔记（4）-高级映射之一对多映射

Mybatis学习笔记（5）-高级映射之多对多映射

假设咱们如今有这样的需求，想知道某一位顾客购买了哪些商品，可是经过观察customers表和products表，咱们发现这两张表并无关联，那么怎么办呢？经过前面的例子，咱们能够发现，经由orders和orderitems这两张表，customers表和products表能创建起间接联系。

咱们以customer表做为主表，那么查询的语句应该是

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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根据上一节的解释，若是咱们使用resultType来定义返回类型，那么定义的POJO类将会包含不少类似的部分，因此最好仍是采用resultMap来定义返回的类型，这样咱们就能够在原有的Customer类上扩展属性来存储订单，那么订单属性内部就会包含订单项，而订单项就会跟具体的商品关联，关联关系看上面的图很容易明白。

那么咱们具体来看看类被咱们定义成什么样子了。

POJO定义

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用于存储该客户所下的订单
    List<Orders> ordersList;
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Orders类中咱们把原先的Customer属性删去，别的都不动

Orders中包含OrderItems属性，OrderItems这么定义：

public class OrderItems {
    private Integer orderNum;
    private Integer orderItem;
    private String prodId;
    private Integer quantity;
    private Double itemPrice;
    //新增的商品类，那么就能够看到订单项中包含的商品的具体信息
    private Products products;
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定义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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咱们从resultMap的定义中就能够看出从Customer到Products的嵌套关系。下面继续看查询语句定义：

<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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经过一系列内链接，查询出与customer有关联的products。

定义接口：

public List<Customer> findCustomerProducts();
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测试代码：

@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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查询结果

从图中能够发现，在customer的orderList属性下有2个订单，每一个订单里会包含一个或多个订单项orderItems，每一个orderItem又会对应一个product。

这就是所谓的多对多查询，嵌套关系多一些。