MyBatis 中一对一和一对多的映射关系

1 一对一映射

比如每位学生有一个地址。

public class Address
    {
        private Integer addrId;
        private String street;
        private String city;
        private String state;
        private String zip;
        private String country;
        // setters & getters
    }

public class Student
    {
        private Integer studId;
        private String name;
        private String email;
        private PhoneNumber phone;
        private Address address;
        //setters & getters
    }

我们根据学生 ID 选择学生信息

方法一：使用句点符号表示嵌套对象的引用，

Student 的 address 属性使用了圆点记法被赋上了 address 对应列的值。

方法二：使用mybatis提供的一对一映射，关键字：association

1）

<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>

元素<association>被用来导入“有一个”(has-one)类型的关联。

如果内嵌的对象有对应的 resultMap 那么使用 <association property="address" resultMap="AddressResult" /> 比较方便。如果内嵌对象需要其它的查询来生成对象，使用方法 3）比较好。

2）也可以使用<association 定义内联的 resultMap，代码如下所示：

<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>

3）通过使用嵌套 select 查询来获取 Student 及其 Address 信息，代码如下：

<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 findStudentWithAddress 加载 student 信息，而第二个调用 findAddressById 来加载 address 信息。

Addr_id 列的值将会被作为输入参数传递给 selectAddressById 语句；如果有多个参数可以通过逗号分隔。

2 一对多映射，关键词使用 <collection>

比如老师和课程的关系，老师可以讲多门课程，每门课程有一个老师

public class Course
    {
        private Integer courseId;
        private String name;
        private String description;
        private Date startDate;
        private Date endDate;
        private Integer tutorId;
        //setters & getters
    }

public class Tutor
    {
        private Integer tutorId;
        private String name;
        private String email;
        private Address address;
        private List<Course> courses;
        // setters & getters
    }

1）使用内嵌结果 ResultMap 实现一对多映射。下面是配置文件

<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>

<collection>元素的 resultMap 属性设置成了 CourseResult，CourseResult 包含了 Course 对象属性与表列名之间的映射。

2）使用嵌套 Select 语句实现一对多映射，配置文件如下

<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>