resultMap如何处理复杂映射问题_Java

resultmap复杂映射问题

association:关联（多对一的情况）
collection: 集合（一对多的情况）
javatype: 用来指定实体类中属性的类型。
oftype: 用来指定映射到list或集合中pojo的类型，泛型的约束类型。

ⅰ 多对一查询：学生——老师

数据库表：

create table `teacher` (
`id` int(10) not null,
`name` varchar(30) default null,
primary key (`id`)
) engine=innodb default charset=utf8;

insert into teacher(`id`, `name`) values (1, '王老师');

create table `student` (
`id` int(10) not null,
`name` varchar(30) default null,
`tid` int(10) default null,
primary key (`id`),
key `fktid` (`tid`),
constraint `fktid` foreign key (`tid`) references `teacher` (`id`)
) engine=innodb default charset=utf8;


insert into `student` (`id`, `name`, `tid`) values ('1', '小明', '1');
insert into `student` (`id`, `name`, `tid`) values ('2', '小红', '1');
insert into `student` (`id`, `name`, `tid`) values ('3', '小张', '1');
insert into `student` (`id`, `name`, `tid`) values ('4', '小李', '1');
insert into `student` (`id`, `name`, `tid`) values ('5', '小王', '1');

(1) 创建实体类pojo

@data
public class student {
    private int id;
    private string name;
    private teacher teacher;
}

@data
public class teacher {
    private int id;
    private string name;
}

(2) 创建学生实体类对应的接口

public interface studentmapper {

    //查询所有学生的信息
    list<student> getstudent();
    list<student> getstudent2();
}

(3) 编写学生接口对应的mapper.xml

为了达到和接口在同一个包中的效果，在resource文件夹下新建包结构com.glp.dao:

<?xml version="1.0" encoding="utf-8" ?>
<!doctype mapper
        public "-//mybatis.org//dtd config 3.0//en"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.glp.dao.studentmapper">

<!--按照结果查询——联表查询-->
    <select id="getstudent2" resultmap="studentmap2">
         select s.id sid,s.name sname,t.name tname from student s, teacher t where s.tid=t.id;
    </select>

    <resultmap id="studentmap2" type="student">
         <result property="id" column="sid"/>
         <result property="name" column="sname"/>

        <association property="teacher" javatype="teacher">
            <result property="name" column="tname"/>
        </association>
    </resultmap>


    <!--按照查询嵌套处理——子查询-->
        <select id="getstudent" resultmap="studentmap" >
           select * from student;
        </select>

    <resultmap id="studentmap" type="student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--复杂属性：对象association, 集合collection-->
        <association property="teacher" column="tid" javatype="teacher" select="getteacher"/>
    </resultmap>
        
        <select id="getteacher" resulttype="teacher">
            select * from teacher where id = #{id};
        </select>

</mapper>

在多对一查询中，需要用到teacher这个表，每个学生都对应着一个老师。而property只能处理单个属性，像teacher这种复杂属性（内含多个属性）需要进行处理。处理复杂对象要用association 。

方式一：

联表查询（直接查出所有信息，再对结果进行处理）

   <resultmap id="studentmap2" type="student">
         <result property="id" column="sid"/>
         <result property="name" column="sname"/>

        <association property="teacher" javatype="teacher">
            <result property="name" column="tname"/>
        </association>
    </resultmap>

直接查询出学生和老师，然后用association去取老师里面的属性property。

方式二：

子查询（先查出学生信息，再拿着学生中的tid，去查询老师的信息）

  <resultmap id="studentmap" type="student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--复杂属性：对象association-->
        <association property="teacher" column="tid" javatype="teacher" select="getteacher"/>
    </resultmap>
        
        <select id="getteacher" resulttype="teacher">
            select * from teacher where id = #{id};
        </select>

在resultmap中引入属性association，通过javatype指定property="teacher"的类型，javatype="teacher"。通过select引入子查询（嵌套查询）。

这里是拿到学生中的tid，去查找对应的老师。

（4）在核心配置类中引入mapper

db.properties:数据库连接参数配置文件

driver = com.mysql.jdbc.driver
url=jdbc:mysql://localhost:3306/mybatis?usessl=true&useunicode=true&chracterencoding=utf8
username =root
password =mysql

mybatis.xml:

<?xml version="1.0" encoding="utf-8" ?>
<!doctype configuration
        public "-//mybatis.org//dtd config 3.0//en"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>


    <properties resource="db.properties">
        <property name="username" value="root"/>
        <property name="password" value="mysql"/>
    </properties>

    <settings>
        <setting name="logimpl" value="stdout_logging"/>
    </settings>

    <typealiases>
        <typealias type="com.glp.pojo.student" alias="student"/>
        <typealias type="com.glp.pojo.teacher" alias="teacher"/>
    </typealiases>


    <environments default="development">

        <environment id="development">
            <transactionmanager type="jdbc"/>
            <datasource type="pooled">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </datasource>
        </environment>

    </environments>

    <mappers>
        <mapper class="com.glp.dao.studentmapper"/>
        <mapper class="com.glp.dao.teachermapper"/>
    </mappers>

</configuration>

注意：

要保证接口和mapper.xml都在同一个包中。

(5) 测试

public class userdaotest {
    @test
    public void getstudent(){
        sqlsession sqlsession = myutils.getsqlsession();
        studentmapper mapper = sqlsession.getmapper(studentmapper.class);
        list<student> list = mapper.getstudent();

        for (student stu:list ) {
            system.out.println(stu);
        }
        sqlsession.close();
    }

    @test
    public void getstudent2(){
        sqlsession sqlsession = myutils.getsqlsession();
        studentmapper mapper = sqlsession.getmapper(studentmapper.class);

        list<student> list = mapper.getstudent2();

        for (student stu:list ) {
            system.out.println(stu);
        }
        sqlsession.close();
    }
}

ⅱ 一对多查询：老师——学生

（1）实体类

@data
public class student {
    private int id;
    private string name;
    private int tid;
}

@data
public class teacher {
    private int id;
    private string name;
    private list<student> students;
}

（2）接口

package com.glp.dao;

public interface teachermapper {

    teacher getteacher(@param("tid") int id);

    teacher getteacher2(@param("tid") int id);
}

（3）接口对应的mapper.xml

<?xml version="1.0" encoding="utf-8" ?>
<!doctype mapper
        public "-//mybatis.org//dtd config 3.0//en"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">


<mapper namespace="com.glp.dao.teachermapper">

  <!--方式一          =======================                  -->
    <select id="getteacher" resultmap="teacherstudent">
        select s.id sid, s.name sname, t.name tname, t.id tid from
        student s ,teacher t where s.tid = t.id and t.id = #{tid};
    </select>

    <resultmap id="teacherstudent" type="teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" oftype="student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultmap>


    <!--方式二          =======================                  -->

    <select id="getteacher2" resultmap="teacherstudent2">
        select * from teacher where id = #{tid};
         <!--这里的tid和接口中指定的属性名相同-->
    </select>

    <resultmap id="teacherstudent2" type="teacher">
	    <result property="id" column="id"/>
        <result property="name" column="name"/>
           <!--上面的两个可以省略-->
        <collection property="students"  column="id" javatype="arraylist" oftype="student"  select="getstubyid"/>
    </resultmap>

    <select id="getstubyid" resulttype="student">
        select * from student where tid=#{tid};
           <!--查询老师对应的学生，#{tid}-->
    </select>
</mapper>

方式一：

联表查询，需要写复杂sql
collection 用来处理集合，oftype用来指定集合中的约束类型
联合查询时，查询出所以结果，然后再解析结果中的属性，将属性property赋予到collection中。

方式二：

子查询，需要写复杂映射关系

查询学生时，需要拿着老师的id去查找，column用来给出老师的id。

（4）测试：

package com.glp.dao;
public class userdaotest {

    @test
    public void getteacher(){
        sqlsession sqlsession = myutils.getsqlsession();
        teachermapper mapper = sqlsession.getmapper(teachermapper.class);

        teacher teacher = mapper.getteacher(1);

        system.out.println(teacher);

        sqlsession.close();
    }


    @test
    public void getteacher2(){
        sqlsession sqlsession = myutils.getsqlsession();
        teachermapper mapper = sqlsession.getmapper(teachermapper.class);

        teacher teacher = mapper.getteacher2(1);

        system.out.println(teacher);

        sqlsession.close();
    }
}