Mybatis中的collection和association一关系

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>