Spring 数据 jpa 按查询计数

Question

I have users containing set of courses and U need to just get the count of courses a student is enrolled into. I do not want ti load the student since this will load the entire student graph object with othe attributed like address etc etc. Is there a way using spring data jpa to get just the count.

Answer 1

You can add a method such as below in your StudentRepository (assuming your entity Student pk as id and set property name as courses)

@Query("select size(s.courses) from Student s where s.id=:id")
long countCoursesByStudentId(@Param("id") long id);

Alternatively you can also add a count method as below in your CourseRepository (Assuming ManyToOne Relation of Course to Student, pk and name of property as id and student)

long countByStudentId(long id);

Answer 2

as you have N to Many relations, you can use size() function for courses from the user.

public class UserIdCountCourses {
    private Long userId;
    private Integer countCourses;

    public UserIdCountCourses(Long userId, Integer countCources) {
        this.userId = userId;
        this.countCourses = countCources;
    }

    public Long getUserId() {
        return userId;
    }

    public Integer getCountCourses() {
        return countCourses;
    }
}

@Query("select new package.....UserIdCountCourses(u.id , size(u.cources)) 
                                           from User u group by u.id")
List<UserIdCountCourses> findUserIdAndCountEnrolledCourses ();

Also, you can use a native query to select only that you need. Native query result is array of objects, but you can apply @SqlResultSetMapping for named native query like (add SqlResultSetMapping into entity , or in xml config file):

@SqlResultSetMapping(
    name="UserIdCountCoursesMapping",
    classes={
        @ConstructorResult(
            targetClass=UserIdCountCourses.class,
            columns={
                @ColumnResult(name="user_id"),
                @ColumnResult(name="count_courses")
            }
        )
    }
)
--just query example
@NamedNativeQuery(name="getUserIdCountCourses", query="SELECT user_id,count (1) FROM user LEFT JOIN cources cu ON user_id=cu.user_id",resultSetMapping="UserIdCountCoursesMapping")