11.一对多处理

11.一对多处理

测试环境搭建

1. 导入lombok

   <dependencies>
   	<!--Lombok-->
   	<dependency>
   		<groupId>org.projectlombok</groupId>
   		<artifactId>lombok</artifactId>
   		<version>1.18.10</version>
   	</dependency>
   </dependencies>
   

2. 新建实体类teacher和student

   package com.believe.pojo;
   
   import lombok.Data;
   
   import java.util.List;
   
   @Data
   public class Teacher {
       private int id;
       private String name;
       private List<Student> students;
   }
   

   package com.believe.pojo;
   
   import lombok.Data;
   
   @Data
   public class Student {
       private int id;
       private String name;
       private int tid;
   }
   

3. 新建Mapper接口

   package com.believe.mapper;
   
   public interface TeacherMapper {
   }
   

   package com.believe.mapper;
   
   public interface StudentMapper {
   }
   

4. 新建Mapper.xml文件

   <?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.believe.mapper.TeacherMapper">
   
   </mapper>
   

   <?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.believe.mapper.StudentMapper">
   
   </mapper>
   

5. 在mybatis-config绑定Mapper.xml

   <!--绑定接口-->
   <mappers>
       <mapper resource="com/believe/mapper/StudentMapper.xml"/>
       <mapper resource="com/believe/mapper/TeacherMapper.xml"/>
   </mappers>
   

6. 测试

SQL语句

select t.id tid,t.name tname,s.id sid,s.name sname from teacher t,student s where t.id = s.tid and t.id = #{tid}

按照查询嵌套处理

<select id="getTeacher01" resultMap="getTeacher01" >
    select * from teacher where id = #{tid}
</select>
<resultMap id="getTeacher01" type="Teacher" >
    <result property="id" column="id"/>
    <collection property="students" column="id" javaType="ArrayList" ofType="Student" select="getStudent01" />
</resultMap>
<select id="getStudent01" resultType="Student" >
    select * from student where tid = #{id}
</select>

按照结果嵌套处理

<select id="getTeacher02" resultMap="getTeacher02" >
    select t.id tid,t.name tname,s.id sid,s.name sname
    from teacher t,student s
    where t.id = s.tid and t.id = #{tid}
</select>
<resultMap id="getTeacher02" 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>

总结

1. 关联—association(多对一)
2. 集合—collection(一对多)
3. javaType&ofType
   1. javaTypre用来指定实体类中属性的类型
   2. ofType用来指定映射到List或者集合中的pojo类型，泛型中的约束类型

注意

1. 保证sql的可读性，尽量保证通俗易懂
2. 注意一对多和多对一中，属性名和字段名的问题
3. 如果问题不好排查错误，可以使用日志工厂排错

原文地址：https://www.cnblogs.com/thetree/p/11_mybatis.html