springboot整合JPA 多表关联 ：一对多 多对多

补充一下自定义SQL

 这是连表查询，可以任意查出字符，用Map接收

  @Test
    void test3() {
        JPAQueryFactory jpaQueryFactory = new JPAQueryFactory(em);
        QStudent student = QStudent.student;
        QMessage message = QMessage.message;
        //constructor(StuMesDto.class, student.id, student.name, student.grade, message.id, message.info)
        List<Map<Expression<?>, ?>> fetch = jpaQueryFactory.select(Projections.map(student.id, student.name, student.gra