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

  @Testvoid 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.grade, message.id, message.info)).from(student).innerJoin(message).on(student.id.eq(message.stu_id)).orderBy(message.id.desc()).limit(1).offset((1 - 1) * 1).fetch();Object o = fetch.get(0).get(message.info);System.out.println(o);}

 这是连表查询，可以任意查出字符，用自定义对象接收

 @Testvoid test3() {JPAQueryFactory jpaQueryFactory = new JPAQueryFactory(em);QStudent student = QStudent.student;QMessage message = QMessage.message;//List<StuMesDto> fetch = jpaQueryFactory.select(Projections.constructor(StuMesDto.class, student.id, student.name, student.grade, message.id, message.info)).from(student).innerJoin(message).on(student.id.eq(message.stu_id)).orderBy(message.id.desc()).limit(1).offset((1 - 1) * 1).fetch();StuMesDto o = fetch.get(0);System.out.println(o);}

package com.jmj.springDataApp.pojo;import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;import javax.persistence.Column;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;@Data
@AllArgsConstructor
@NoArgsConstructor
public class StuMesDto {private Long sid;private String name;private Integer grade;private Long id;private String info;
}

QTeacher teacher = QTeacher.teacher;
QStudent student = QStudent.student;List<TeacherWithStudentsDto> result = jpaQueryFactory.select(Projections.constructor(TeacherWithStudentsDto.class,teacher.id,teacher.name,Projections.list(student))).from(teacher).leftJoin(student).on(teacher.id.eq(student.teacher.id)).groupBy(teacher.id).orderBy(teacher.id.asc()).limit(100) // 限制查询结果为前100个老师.fetch().stream().peek(dto -> dto.setStudents(dto.getStudents().subList(0, Math.min(dto.getStudents().size(), 10)))).collect(Collectors.toList());