MyBatis结果集映射(ResultMap)

Mybatis的查询可以将查询出的结果集转换成Java对象。ResultMap有以下几种用途：

1. 如果数据表的列名和Java对象的属性名不一致，在ResultMap中可以进行关联
2. 定义一对一的关联
3. 定义一对多的关联

第一个用途很好理解。
第二个用途举例说明。假设我们有一张学生表信息Students（studId,name,email,addressId），以及一张学生地址表StuAddress（addressId,country,province,city,details），Students(addressId)关联StuAddress(addressId)。我们可以这样来定义两个Java类

public class Student {
    private int studId;
    private String name;
    private String email;
    private int addressId;
    //一对一的关联关系
    private StuAddress address;
    // getters and setters......
}

public class StuAddress {
    private int addressId;
    private String country;
    private String province;
    private String city;
    private String details;
    // getters and setters......
}

如果我们进行一个连接查询时，配置一下ResultMap，就可以将结果集直接映射到Student对象，该对象具有学生信息和地址信息，后面我将会介绍一对一映射的方式。

第三个用途举例说明。如果我们还有一个课程表Course（courseId,courseName,teacherName）以及一个学生选课表StuCourse(id,courseId,studentId)，Course表保存所有的课程，而StuCourse表保存学生与课程的关联关系，这种关联关系是一对多。那么，我们需要一个Course类

public class Course {
    private int courseId;
    private String courseName;
    private String teacherName;
    //getters and setters......
}

我们的Student类也要加上一个关联属性，添加后我们的Student类就变成了这样

public class Student {
    private int studId;
    private String name;
    private String email;
    private int addressId;
    //一对一的关联关系
    private StuAddress address;
    //一对多的关联关系
    private List courses;
    // getters and setters......
}

接下来我们来看看ResultMap的使用细节。

简单的用法

<resultMap id="StudentResult" type="Student">
    
    <id property="studId" column="stud_id"/>
    <result property="name" column="name"/>
    <result property="email" column="email"/>
resultMap>
<select id="findAllStudents" resultMap="StudentResult" >
    SELECT * FROM STUDENTS
select>

如果想使用某个ResultMap作为返回值，则在select节点中应该使用resultMap而不是resultType。在节点内，如果子节点为主键，则应该使用，而不是。

一对一的映射

"Student" id="StudentWithAddressResult">
    <id property="studId" column="stud_id"/>
    <result property="name" column="name"/>
    <result property="email" column="email"/>
    <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"/>

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}

我们也可以使用嵌套的方式来指定一对一的映射关系

"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"/>

"Student" id="StudentWithAddressResult">
    <id property="studId" column="stud_id"/>
    <result property="name" column="name"/>
    <result property="email" column="email"/>
    property="address" resultMap="AddressResult"/>

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}

或者可以这样写

"Student" id="StudentWithAddressResult">
    <id property="studId" column="stud_id"/>
    <result property="name" column="name"/>
    <result property="email" column="email"/>
    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"/>
    

一对多的映射

<resultMap type="Course" id="CourseResult">
    <id column="course_id" property="courseId"/>
    <result column="name" property="name"/>
    <result column="description" property="description"/>
    <result column="start_date" property="startDate"/>
    <result column="end_date" property="endDate"/>
resultMap>
<resultMap type="Tutor" id="TutorResult">
    <id column="tutor_id" property="tutorId"/>
    <result column="tutor_name" property="name"/>
    <result column="email" property="email"/>
    
    <collection property="courses" resultMap="CourseResult"/>
resultMap>
<select id="findTutorById" parameterType="int" resultMap="TutorResult">
    SELECT T.TUTOR_ID, T.NAME AS TUTOR_NAME, EMAIL, C.COURSE_ID,
    C.NAME, DESCRIPTION, START_DATE, END_DATE
    FROM TUTORS T LEFT OUTER JOIN ADDRESSES A ON T.ADDR_ID=A.ADDR_ID
    LEFT OUTER JOIN COURSES C ON T.TUTOR_ID=C.TUTOR_ID
    WHERE T.TUTOR_ID=#{tutorId}
select>