[置顶] Mybatis(七)- 一对一映射

一对一映射

举个例子来说我们的Student和我们的Address是一对一的关系

<resultMap type="Student" id="StudentWithAddressResult">
<id property="studId" column="stud_id" />
<result property="name" column="name" />
<result property="email" column="email" />
<result property="phone" column="phone" />
<result property="address.addrId" column="addr_id" />
<result property="address.street" column="street" />
<result property="address.city" column="city" />
<result property="address.state" column="state" />
<result property="address.zip" column="zip" />
<result property="address.country" column="country" />
</resultMap>
<select id="selectStudentWithAddress" parameterType="int"
resultMap="StudentWithAddressResult">
SELECT STUD_ID, NAME, EMAIL, A.ADDR_ID, STREET, CITY, STATE,
ZIP, COUNTRY
FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON
S.ADDR_ID=A.ADDR_ID
WHERE STUD_ID=#{studId}
</select>

对于一对一映射上述是最简单的配置。

使用嵌套结果association（有一个）

<resultMap type="Address" id="AddressResult">
<id property="addrId" column="addr_id" />
<result property="street" column="street" />
<result property="city" column="city" />
<result property="state" column="state" />
<result property="zip" column="zip" />
<result property="country" column="country" />
</resultMap>
<resultMap type="Student" id="StudentWithAddressResult">
<id property="studId" column="stud_id" />
<result property="name" column="name" />
<result property="email" column="email" />
<association property="address" resultMap="AddressResult" />
</resultMap>
<select id="findStudentWithAddress" parameterType="int"
resultMap="StudentWithAddressResult">
SELECT STUD_ID, NAME, EMAIL, A.ADDR_ID, STREET, CITY, STATE,
ZIP, COUNTRY
FROM STUDENTS S LEFT OUTER JOIN ADDRESSES A ON
S.ADDR_ID=A.ADDR_ID
WHERE STUD_ID=#{studId}
</select>

上述展示了association的用法，主要表示has a

使用嵌套结果association的用法

<resultMap type="Address" id="AddressResult">
<id property="addrId" column="addr_id" />
<result property="street" column="street" />
<result property="city" column="city" />
<result property="state" column="state" />
<result property="zip" column="zip" />
<result property="country" column="country" />
</resultMap>
<select id="findAddressById" parameterType="int"
resultMap="AddressResult">
SELECT * FROM ADDRESSES WHERE ADDR_ID=#{id}
</select>
<resultMap type="Student" id="StudentWithAddressResult">
<id property="studId" column="stud_id" />
<result property="name" column="name" />
<result property="email" column="email" />
<association property="address" column="addr_id" select="findAddressById" />
</resultMap>
<select id="findStudentWithAddress" parameterType="int"
resultMap="StudentWithAddressResult">
SELECT * FROM STUDENTS WHERE STUD_ID=#{Id}
</select>

在此方式中，<association>元素的 select 属性被设置成了 id 为 findAddressById 的语句。这里，两个分开的SQL 语句将会在数据库中执行，第一个调用 findStudentById 加载 student 信息，而第二个调用 findAddressById 来加载 address 信息。列为addr_id的值作为参数传递进去。

嵌套结果和嵌套查询，嵌套查询会引发我们的N+1问题，防止我们N+1问题我们可以使用嵌套结果。