Mybatis中的collection和association一关系

时间:2023-03-08 22:02:06

collection 一对多和association的多对一关系

学生和班级的一对多的例子

班级类:

package com.glj.pojo;

import java.io.Serializable;
import java.util.List; public class Clazz implements Serializable{
private Integer id;
private String code;
private String name;
//班级与学生是一对多的关系
private List<Student> students;
//省略set/get方法
}

学生类:

package com.glj.pojo;

import java.io.Serializable;

public class Student implements Serializable {
private Integer id;
private String name;
private String sex;
private Integer age;
//学生与班级是多对一的关系
private Clazz clazz;
//省略set/get方法
}

ClazzMapper使用到了集合-collection 即为一对多,一个班级面对多个学生

package com.glj.mapper;

import com.glj.pojo.Clazz;

public interface ClazzMapper {
Clazz selectClazzById(Integer id);
}
<?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="com.glj.mapper.ClazzMapper">
<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
select * from tb_clazz where id = #{id}
</select>
<resultMap type="com.glj.pojo.Clazz" id="clazzResultMap">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
<!-- property: 指的是集合属性的值, ofType:指的是集合中元素的类型 -->
<collection property="students" ofType="com.glj.pojo.Student"
column="id" javaType="ArrayList"
fetchType="lazy" select="com.glj.mapper.StudentMapper.selectStudentByClazzId">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
</collection>
</resultMap>
</mapper>

StudentMapper则是与班级为多对一关系,所以使用了关联-association

package com.glj.mapper;

import com.glj.pojo.Student;

public interface StudentMapper {
Student selectStudentById(Integer id);
}
<?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="com.glj.mapper.StudentMapper">
<select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
select * from tb_clazz c,tb_student s where c.id = s.id and s.id = #{id}
</select>
<select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
select * from tb_student where clazz_id = #{id}
</select>
<resultMap type="com.glj.pojo.Student" id="studentResultMap">
<id property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="age" column="age"/>
<association property="clazz" javaType="com.glj.pojo.Clazz">
<id property="id" column="id"/>
<result property="code" column="code"/>
<result property="name" column="name"/>
</association>
</resultMap>
</mapper>

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