Really confused by how one to many works in JPA, all the documents that I read, uses both one to many and many to one in their example, and I don't know if they are necessary or not, and it doesn't work when I tried it.
My question is, suppose I have two tables, and I want to populate the College object using findCollegeData() method, so that all the student in this college are in a list when I initialize the object.
Below is my approach, I am able to store all the students in the college list using storeCollegeData() method, but I am not able to retrieve the college object fully, the student list is always empty, even though the data is in the database, and it works if I try to search for student using college name directly.
public static EntityManager entityManager = something;
@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public College {
@Id
@GeneratedValue(strategy= GenerationType.AUTO)
private int cId;
private String collegeName;
private int numOfStudent;
@OneToMany(mappedBy="collegeName", cascade=CascadeType.ALL, orphanRemoval=true)
private List<Student> studentList = new ArrayList<>();
}
@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public Student {
@Id
@GeneratedValue(strategy= GenerationType.AUTO)
private int sId;
private String name;
private String collegeName;
private String city;
}
// college.getStudentList is always empty and I don't know why
public findCollegeData(String collegeName) {
College college = entityManager.find(College.class, collegeName);
}
// Student data in the studentList are inserted into student table
public storeCollegeData(College college) {
entityManager.persist(college);
}
// This method works
public findStudent(String collegeName) {
CriteriaBuilder cb = provider.get().getCriteriaBuilder();
CriteriaQuery<Student> query = cb.createQuery(Student.class);
Root<Student> student = query.from(Student.class);
query.where(
cb.and(
cb.equal(student.get("collegeName"), collegeName)
)
);
JobStatisticDB Student = provider.get().createQuery(query).getSingleResult();
}
Am i missing something??? Is join more appropriate than map here??? I dont know wat to do man
EDITED: Got it to work by changing both of the collegeName as the primary key of table by adding @Id annotation, however though, how can I add an sId and cId to the table, so they can have duplicate college name???? Right now, I can't have duplicate college with the same name, and student that that goes to the same college!
Final Edited: Changed database design to use foreign key see solution below
The field you reference in mappedBy
must contain a value that equates to College
's id field. Change it to collegeName
instead of city
, and it should work.
The accepted answer is incorrect: you define relationships between entities . The mappings should be as below for a bi-directional @OneToMany
College:
@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public College {
@Id
@GeneratedValue(strategy= GenerationType.AUTO)
private int cId;
private String collegeName;
private int numOfStudent;
@OneToMany(mappedBy="college", cascade=CascadeType.ALL, orphanRemoval=true)
private List<Student> studentList = new ArrayList<>();
}
Student:
@Entity
@Data
@NoArgsConstructor
@AllArgsConstructor
public Student {
@Id
@GeneratedValue(strategy= GenerationType.AUTO)
private int sId;
private String name;
private String city;
//student table has a FK column college_id
@ManyToOne
@JoinColumn(name = "college_id")
private College college;
}
EntityManager find() takes the PK as an argument:
public findCollege(int collegeId) {
College college = entityManager.find(College.class, collegeId);
college.getStudents(); //will be populated
}
public findStudent(int studentId) {
Student student = entityManager.find(Student.class, studentId);
student.getCollege(); //will be populated
student.getCollege().getStudents(); //will be populated
}
If you want to find a college by name create a JPQL or Criteria query:
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