Mybatis association分布查询为单个基本类型字段（非自定义对象字段）赋值

之前使用association主要是下面两种情况

1. 左连接之后封装对象
2. 分步查询封装对象

比如

左连接查询之后封装Order里面的Customer对象

<!--由于数据库字段和实体类字段名称不对应，所以设置resultMap进行映射-->
    <resultMap id="OrderMap" type="Order">
        <id property="orderId" column="order_id"/>
        <result property="orderName" column="order_name"/>
        <result property="orderNum" column="order_num"/>
        <association property="customer" javaType="Customer">
            <id column="cust_id" property="custId"></id>
            <result column="cust_name" property="custName"></result>
            <result column="cust_phone" property="custPhone"></result>
            <result column="email" property="custEmail"></result>
            <result column="cust_profession" property="custProfession"></result>
        </association>
    </resultMap>
 
 	<select id="getAllOrder" resultMap="OrderMap">
     	select * from `order` as o left join `customer` as c on o.cust_id = c.cust_id
 	</select>

给association传入cust_id再进行下一次查询，查出来的对象封装成Customer对象

<!--分步查询(就是利用第一次查询的结果进行下一次查询)-->
    <resultMap id="OrderMap2" type="Order">
        <id property="orderId" column="order_id"/>
        <result property="orderName" column="order_name"/>
        <result property="orderNum" column="order_num"/>
        <association property="customer" javaType="Customer"
                     select="fei.mapper.CustomerMapper.queryCustomerById" column="cust_id">
            <id column="cust_id" property="custId"></id>
            <result column="cust_name" property="custName"></result>
            <result column="cust_phone" property="custPhone"></result>
            <result column="email" property="custEmail"></result>
            <result column="cust_profession" property="custProfession"></result>
        </association>
    </resultMap>

 <!--根据cust_id查询客户-->
    <select id="queryCustomerById" parameterType="Integer" resultMap="CustomerMap">
	  SELECT * FROM `customer` WHERE cust_id  = #{cust_id}
	</select>

这两种比较常见的association用法相信大家都很熟悉
下面是我经过探索发现，association可以分步查询为单个基本类型字段（非自定义对象字段）赋值

---

这样一个需求，Customer对象有一个Integer类型的orderNum字段（订单数），现在可以直接使用association可以分步查询，然后为orderNum字段赋值

代码如下

<!--分步查询(就是利用第一次查询的结果进行下一次查询)-->
   <resultMap id="CustomerMap" type="Customer">
        <!--主键的映射-->
        <id column="cust_id" property="custId"></id>
        <result column="cust_name" property="custName"></result>
        <result column="cust_phone" property="custPhone"></result>
        <result column="email" property="custEmail"></result>
        <result column="cust_profession" property="custProfession"></result>
        <association property="orderNum" javaType="Integer"
                     select="fei.mapper.OrderMapper.queryOrderNum" column="cust_id">
        </association>
    </resultMap>

然后fei.mapper.OrderMapper.queryOrderNum定义为

 <!--根据cust_id查询订单数量-->
    <select id="queryCustomerById" resultType="Integer">
	  SELECT COUNT(order_id) FROM `order` WHERE cust_id  = #{cust_id}
	</select>

完成！！

association分布查询为单个基本类型字段（非自定义对象字段）赋值