MyBatis mapper.xml配置一对一关系映射的几种方式整理

所需的类:

public class Student {
private Integer studId;
private String name;
private String email;
private Date dob;
private Address address;
//get and set method
......
}

public class Address {
private Integer addrId;
private String street;
private String city;
private String state;
private String zip;
private String country;
//get and set method
......
}

mapper.xml配置：

第一种:嵌套(同个xml内)结果 ResultMap

在Student的ResultMap中使用association 标签引用Address的ResultMap的id

......
<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>

<!-- 根据id查询-->
<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=#{Id}
</select>
.....

第二种：使用圆点记法为内嵌的对象的属性赋值

在resultMap中，Student 的address属性使用了圆点记法

被赋上了address对应列的值

......
<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=#{Id}
</select>
......

第三种:使用association定义内联的 resultMap

需注意:association javaType=”Address”

<resultMap type="Student" id="StudentWithAddressResult">
<id property="studId" column="stud_id" />
<result property="name" column="name" />
<result property="email" column="email" />
<association property="address" javaType="Address">
<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" />
</association>
</resultMap>

<select id="findStudentWithAddressByStuId"  parameterType="int"  resultMap="StudentResult">
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=#{Id}
</select>

第四种:嵌套select查询实现一对一关系映射

在此方式中，association 元素的 select属性被设置成了id为 findAddressById 的语句，然后查询结果赋值

......
<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>

<!-- 查询 -->
<select id="findStudentWithAddress" parameterType="int"
resultMap="StudentWithAddressResult">
SELECT * FROM STUDENTS WHERE STUD_ID=#{Id}
</select>
......

参考:Java Persistence with MyBatis 3