mybatis关联关系映射

一、一对多

举个例子讲个痛快：查一个订单对应的多个订单项

贴出了重要代码，OrderMapper.xml 

 <!-- 一对多的关系 -->
 <!-- property: 指的是集合属性的值, ofType：指的是集合中元素的类型 -->

<resultMap id="OrderMap" type="com.zking.ssm.model.Order">
       <result property="orderId" column="order_id"></result>
       <result property="orderNo" column="order_no"></result>
       <collection property="orderItems" ofType="com.zking.ssm.model.OrderItem">
           <result property="orderItemId" column="order_item_id"></result>
           <result property="productId" column="product_id"></result>
           <result property="quantity" column="quantity"></result>
           <result property="oid" column="oid"></result>
       </collection>
   </resultMap>

写SQL语句时要注意 使用左外连接而非内连接！！！

<select id="queryOrderById" resultMap="OrderMap" parameterType="java.lang.Integer">
        select * from t_hibernate_order o,t_hibernate_order_item oi
         where o.order_id= oi.oid
         and o.order_id = #{orderId}
    </select>

测试

    @Autowired
    private OrderService orderService;
    @Test
    public void queryOrderById() {
        List<Order> orders = orderService.queryOrderById(3);
        for (Order order : orders) {
            System.out.println(order);
        }

    }

二、多对一

多个订单项对应的订单

 <!-- 多对一的关系 -->
<!-- property: 指的是属性的值, javaType：指的是属性的类型-->

<resultMap id="OrderItemMap" type="com.zking.ssm.model.OrderItem">
        <result property="orderItemId" column="order_item_id"></result>
        <result property="productId" column="product_id"></result>
        <result property="quantity" column="quantity"></result>
        <result property="oid" column="oid"></result>
        <association property="orders" javaType="com.zking.ssm.model.Order">
            <result property="orderId" column="order_id"></result>
            <result property="orderNo" column="order_no"></result>
        </association>
    </resultMap>

 <select id="queryOrderItemById" resultMap="OrderItemMap" parameterType="java.lang.Integer">
        select * from t_hibernate_order o,t_hibernate_order_item oi
         where o.order_id= oi.oid
         and oi.order_item_id = #{orderId}
    </select>

测试：

  @Autowired
    private OrderItemService orderItemService;
    @Test
    public void queryOrderItemById() {
        OrderItem orderItem = orderItemService.queryOrderItemById(3);
        System.out.println(orderItem);
    }

三、多对多

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多对多相当于两个一对多，同上面的例子一样。

注意：

一对多用ofType，多对一用JavaType。