mybatis级联问题（association、collection和discriminator）

mybatis的级联一般分为三种。一对一，一对多，多对多。但是实际生活中多对多比较少，因为它很复杂，书本上说是建议把多对多分解成双向一对多。接下来开始简介：

association：一对一，比如你这个人和身份证就是一一对应的（我之前的requestMap的博客中简单讲了下这个元素的作用http://blog.csdn.net/zyf2333/article/details/77603028）

collection：一对多，比如一个班级对应着多个学生。

discriminator：鉴别器，根据特定的条件去关联不同的结果集。比如有个Person表。你想实例化一个人，没问题，但是你要根据实际情况来判断是实例化一个男性，还是实例化一个女性。（就类似于c语言的“switch”）

这里主要讲一下collection的一对多如何配置xml。利用书本上的例子来吧

首先每个学生要学习很多门课程，而每门课程又有相对应的课程成绩，因此此处就有了两个级联（学生->课程一对多；课程->课程成绩一对一）。一对一用的association，一对多用的collection。创建课程和成绩的POJO。

学生->课程1成绩->课程1
   ->课程2成绩->课程2
   ->课程3成绩->课程3
  (LectureBean代表课程)
  (StrudentLectureBean代表课程成绩)
  伪代码如下（主要是关注xml怎么配置）：

public class LectureBean{
    private Integer id;
    private String lectureName;
    private String note;
    ...setter and getter...
}

public class StrudentLectureBean{
    private int id;
    private Integer studentID;
    //用来读取课程信息
    private LectureBean lecture;
    private BigDecimal grade;
    private String note;
    ...setter and getter...
}

这里还有个StudentBean

public class StudentBean{
    ...省略，包含了姓名，性别，id等诸多信息
    //用来读入多个课程成绩
    List<StudentLectureBean> studentLectureList=new List<StudentLectureBean>();
    ...省略
}

各个mapper.xml配置清单如下。看着代码来理解吧：
StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>   
<!DOCTYPE mapper   
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"   
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="domain.blog.mappers.AuthorMapper">

    <!-- 先设定resultMap -->
    <resultMap type="com.learn.chapter4.po.StudentBean" id="studentMap">
        <id property="id" column="id" />
        <result property="cnname" column="cnname" />
        <result property="sex" column="sex" jdbcType="INTEGER"
            javaType="com.learn.chapter4.enums.SexEnum" />
        <result property="note" column="note" />

        <!-- 接下来是正文！ -->
        <!-- 引用了StudentLectureMapper.xml中的方法 -->
        <collection property="studentLectureList" column="id"
            select="com.learn.chapter4.mapper.StudentLectureMapper.findStudentLectureByStuId"></collection>
    </resultMap>

    <select id="getStudent" parameterType="int" resultMap="studentMap">
        select
        id,cnname,sex,note from t_student where id=#{id}
    </select>
</mapper>  

StudentLectureMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>   
<!DOCTYPE mapper   
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"   
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="domain.blog.mappers.AuthorMapper">

    <!-- 先设定resultMap -->
    <resultMap type="com.learn.chapter4.po.StudentLectureBean"
        id="studentLectureMap">
        <id property="id" column="id" />
        <result property="studentId" column="student_id" />
        <result property="grade" column="grade" />
        <result property="note" column="note" />
        <association property="lecture" column="lecture_id"
            select="com.learn.chapter4.mapper.LectureMapper.getLecture" />
    </resultMap>

    <select id="findStudeentLectureByStuId" parameterType="int"
        resultMap="studentLectureMap">
        select
        id,student_id,lecture_id,grade,note from
        t_student_lecture where id=#{id}
    </select>
</mapper>  

LectureMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>   
<!DOCTYPE mapper   
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"   
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="domain.blog.mappers.AuthorMapper">

    <select id="getLecture" parameterType="int"
        resultType="com.learn.chapter4.po.LectureBean">
        select
        id,lecture_name as lectureName,note from t_lecture
        where id=#{id}
    </select>
</mapper>  

从这里就可看到各个mapper之间是如何关联的了。

接下来看看鉴别器discriminator
假设有一个男女学生的表，我们要根据不同的情况来**不同的实例。
男女学生类伪代码如下

//男学生
public class MaleStudentBean extends StudentBean{
    private List<StudentHealthMaleBean> studentHealthMaleBean=null;
    ...setter and getter
}

//女学生
public class FemaleStudentBean extends StudentBean{
    private List<StudentHealthFemaleBean> studentHealthFemaleBean=null;
    ...setter and getter
}

改动StudentMapper.xml，具体请看代码以及代码注释

<?xml version="1.0" encoding="UTF-8" ?>   
<!DOCTYPE mapper   
    PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"   
    "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="domain.blog.mappers.AuthorMapper">

    <!-- 先设定resultMap -->
    <resultMap type="com.learn.chapter4.po.StudentBean" id="studentMap">
        <id property="id" column="id" />
        <result property="cnname" column="cnname" />
        <result property="sex" column="sex" jdbcType="INTEGER"
            javaType="com.learn.chapter4.enums.SexEnum" />
        <result property="note" column="note" />
        <collection property="studentLectureList" column="id"
            select="com.learn.chapter4.mapper.StudentLectureMapper.findStudentLectureByStuId"></collection>

        <!-- 开始写鉴别器！此处根据不同的case，引用不同的resultMap，就写在下方 -->
        <discriminator javaType="int" column="sex">
            <case value="1" resultMap="maleStudentMap" />
            <case value="2" resultMap="femaiStudentMap" />
        </discriminator>

    </resultMap>

    <!-- 写上面不同case对应的reusltMap 先男生再女生-->
    <resultMap type="com.learn.chapter4.po.MaleStudentBean" id="maleStudentMap"
        extends="studentMap">
        <collection property="studentHealthMaleList（此处是另外关联的表，不用关心了）"
            select="...略" />
    </resultMap>
    <resultMap type="com.learn.chapter4.po.FemaleStudentBean" id="femaleStudentMap"
        extends="studentMap">
        <collection property="studentHealthMaleList（此处是另外关联的表，不用关心了）"
            select="...略" />
    </resultMap>
    <!-- 结束 -->

    <select id="getStudent" parameterType="int" resultMap="studentMap">
        select
        id,cnname,sex,note from t_student where id=#{id}
    </select>
</mapper>