如何在MyBatis（Spring）映射器中使用表上的select创建一个对象（Spring）？

Question

I am new to MyBatis and Spring. I have two SQL tables, let's say a list of Universities and a list of Students.

UNIVERSITY table has id, name, listOfStudents

STUDENTS table has name_ofStudent, GPA, age, major, id, universityId

I have University.java, Student.java, UniversityManager.java, StudentManager.java (both with crud operations in their manager) (read's parameter is an int id).

If this is my UniversityMapper.xml (below) how do I "SELECT" (by ID) a University? I know how the basic select works but my main question is once I select get by id the name of the university how do I also so get the list of students belonging to that students?

I want to know how to select using mybatis the list of students that match the universityID and create a student object and make it part of the university object.

<?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="java.University">
  <resultMap id="BaseResultMap" type="java.University">
    <id column="ID" jdbcType="VARCHAR" property="id" />
    <result column="NAME" jdbcType="VARCHAR" property="name" />
  </resultMap>
  <insert id="createUniversity" parameterType="java.University">
    insert into DATABASE.University(ID, NAME)
    values (#{id,jdbcType=VARCHAR}, #{name,jdbcType=VARCHAR})
        </insert>

   <select id="getUniversity" parameterType="int" resultMap="java.University">

(This is where I need help)

</select>

   <update id="updateUniversity" parameterType="java.University">
    update DATABASE.University
    set ID= #{id,jdbcType=VARCHAR},
      NAME= #{name,jdbcType=VARCHAR}
  </update>
  <delete id="deleteUniversity" parameterType="java.University">
    delete from DATABASE.University
    where ID= #{id,jdbcType=VARCHAR}
  </delete>
</mapper>

Answer 1

It's all written in the documentation in the part where they mention the Advanced ResultMap s.

If your University class is something like this:

public class University{
    private Long id;
    private String name;
    private List<Student> students;
    // getters, setters, etc.
}

And your Student class is like:

public class Student{
    private String name_ofStudent;
    private Double GPA;
    private Integer age;
    private String major;
    private Long id;
    private Long universityId;
    // getters, setters, etc.
}

You can have query that gets the University and its Students as follows:

<select parameterType="Long" resultMap="UniversityResultMap">
    SELECT U.uni_id, U.uni_name, S.name_ofStudent, S.gpa, S.age, S.major, S.stu_id, S.stu_uni_id
    FROM UNIVERSITY U
    JOIN STUDENT S ON U.ID = S.UNIVERSITYID
    WHERE U.ID = #{id}
</select>

Then you can map the results as:

<resultMap id="StudentResultMap" resultMap="java.Student">
    <result column="NAME_OFSTUDENT" jdbcType="VARCHAR" property="name_ofStudent" />
    <result column="GPA" jdbcType="DECIMAL" property="GPA" />
    <result column="AGE" jdbcType="DECIMAL" property="age" />
    <result column="MAJOR" jdbcType="VARCHAR" property="major" />
    <result column="STU_ID" jdbcType="DECIMAL" property="id" />
    <result column="STU_UNI_ID" jdbcType="DECIMAL" property="universityId" />
</resultMap>

<resultMap id="UniversityResultMap" parameterType="Long" resultMap="java.University">
    <id column="UNI_ID" jdbcType="DECIMAL" property="id" />
    <result column="NAME" jdbcType="VARCHAR" property="name" />
    <collection  property="students" ofType="java.Student" resultMap="StudentResultMap"/>
</resultMap>

If you have a different abstraction in mind about the classes, let us know.