在 JPA 中使用父实体获取子实体

Question

I have these given below entities and I want to execute some queries and I am not sure how to do it in JPA.

School.java

@Data
@Builder
public class School {
    private String schoolName;
    private int schoolId;
    private List<Grade> grades;
}

Grade.Java

@Data
@Builder
public class Grade {
    private String grade;
    private int gradeId;
    private List<Student> students;
}

Student.java

@Data
@Builder
public class Student {
    private String studentName;
    private int studentId;
    private List<Subject> subjects;
}

Subject.java

@Data
@Builder
public class Subject {
    private String subjectName;
    private int subjectId;
}

I need to execute these queries:

1. Get all Students By schoolId
2. Get all Students By gradeId
3. Get all Students By schoolId & gradeId

How can this be achieved?

Answer 1

First, please go through the JPA documentation a bit. Look for Native Queries .

For example for first one:

1) Get all Students By schoolId

@Query("SELECT s FROM Student s WHERE s.school_id = :schoolId")
List<Student> findStudentsBySchoolId(@Param("schoolId") Long schoolId);

Considering you have following:

• A mapping of School in Student with mapping column schoolId .
• And offcourse a repository for Student in which above method will go. You might also want to extend JPARepository .

Also Note

List<Student> findBySchool_SchoolId(Long id);

It is not a very good idea to name your methods with underscore in Java . See this