Mybatis：association【多对一】，collection【一对多】

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本文介绍MyBatis框架中实现一对多和多对一关联映射的方法，包括使用association和collection标签的具体配置，以及如何通过嵌套查询和嵌套结果实现关联对象的数据加载。

association【多对一】

@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Student {
    private int id;
    private String name;
    /**
     * 学生要关联一个老师
     */
    private Teacher teacher;
}
@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Teacher {
    private int id;
    private String name;
}

public interface StudentMapper {

    List<Student> getStudent();

    List<Student> getStudent2();
}

 <resultMap id="StudentTeacher" type="com.sunreal.pojo.Student">
        <result column="id" property="id"></result>
        <result column="name" property="name"></result>
        <association property="teacher" column="tid" javaType="com.sunreal.pojo.Teacher" select="getTeacher"/>
    </resultMap>

    <select id="getStudent" resultMap="StudentTeacher">
        select *
        from student
    </select>

    <select id="getTeacher" resultType="com.sunreal.pojo.Teacher">
        select *
        from teacher
        where id = #{id}
    </select>


    <resultMap id="StudentTeacher2" type="com.sunreal.pojo.Student">
        <result column="sid" property="id"></result>
        <result column="sname" property="name"></result>
        <association property="teacher" javaType="com.sunreal.pojo.Teacher">
            <result property="name" column="tname"></result>
        </association>
    </resultMap>
    <select id="getStudent2" resultMap="StudentTeacher2">
        select s.id sid, s.name sname, t.name tname
        from student s,
             teacher t
        where s.tid = t.id
    </select>

collection【一对多】

@Alias("Student")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Student {
    private int id;
    private String name;
    private int tid;
}
@Alias("Teacher")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Teacher {
    private int id;
    private String name;
    /**
     * 一个老师包含多个学生
     */
    private List<Student> studentList;
}

public interface TeacherMapper {

    Teacher getTeacher(@Param("tid") int id);

    Teacher getTeacher2(@Param("tid") int id);
}

 <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"></result>
        <result property="name" column="tname"></result>
        <collection property="studentList" ofType="Student">
            <result property="id" column="sid"></result>
            <result property="name" column="sname"></result>
            <result property="tid" column="tid"></result>
        </collection>
    </resultMap>
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid, s.name sname, t.name name, t.id tid
        from student s,
             teacher t
        where s.tid = t.id
          and t.id = #{tid}
    </select>

    <resultMap id="TeacherStudent2" type="Teacher">
        <collection property="studentList" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id"/>
    </resultMap>
    <select id="getTeacher2" resultMap="TeacherStudent2">
        select *
        from teacher
        where id = #{tid}
    </select>

    <select id="getStudentByTeacherId" resultType="Student">
        select *
        from student
        where tid = #{tid}
    </select>