Can Spring Data JPA Enforce ManyToOne Relationship via #JoinColumn - spring-boot

Question

I have a Spring Boot application using Spring Data REST and Spring Data JPA. I have two domain entities: Student and Classroom, where many students can belong to the same classroom.
Student:
#Data
#Entity
#Table(name = "STUDENT")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "STUDENT_ID")
private Integer studentId; // This Id has been setup as auto generated in DB
#Column(name = "ROOM_ID")
private Integer roomId;
#ManyToOne
#JoinColumn(name = "ROOM_ID", nullable = false, updatable = false, insertable = false)
private Classroom classroom;
}
Classroom:
#Data
#Entity
#Table(name = "CLASSROOM")
public class Classroom {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "ROOM_ID")
private Integer roomId; // This Id has been setup as auto generated in DB
#OneToMany(mappedBy = "classroom")
private List<Student> studentList;
.....// other fields related to a classroom
}
And the Student repository:
public interface StudentRepository extends CrudRepository<Student , Integer>{
List<Student> findByClassroom(#Param("room") Classroom room);
}
And the Classroom repository:
public interface ClassroomRepository extends CrudRepository<Classroom , Integer>{
}
And I have a SpringApplication main file, but no controller.
There is already one classroom with room id=1 in the CLASSROOM table. When I gave the following request to POST to http://localhost:8080/students, a new student record was created in the Student table, which I expected it to fail because there isn't a classroom with id=100 exists in the CLASSROOM.
So my question is that: can Spring Data JPA enforce a manyToOne relationship or this foreign key enforcement has to be done on the database side (the not-null ROOM_ID column in the Student table is NOT defined as foreign key by our DBA for a legitimate consideration). If it has to be done on the database side, what is the point to define the manyToOne relationship in entity files?
Also, I know that I have redundant classroom fields in the Student entity, I just don't know which one to keep in the Student entity (the roomId or the "classroom" field), because when I create a student, I want to give only the roomId of a classroom in the request. Thanks!
{
"roomId": 100 // I expect this request to fail because no roomId=100 in the CLASSROOM table.
}

Answer 1

what is the point to define the manyToOne relationship in entity files
Because is an Object Relational Mapping tool that allows you define entity graphs.
You are currently passing roomId which in your Entity is just another field so you needs to remove that.
#Entity
#Table(name = "STUDENT")
public class Student {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "STUDENT_ID")
private Integer studentId; // This Id has been setup as auto generated in DB
#ManyToOne
#JoinColumn(name = "ROOM_ID", nullable = false)
private Classroom classroom;
}
In Spring Data Rest you then defined an association by passing the self link of the referenced entity.
Your request then needs to look like the below:
{
"classroom" : "http://localhost:8080/classrooms/1"
}
Also removing the ID as you are POSTing a new record and, as you note, the ID is auto-generated in the database.
See also:
https://www.baeldung.com/spring-data-rest-relationships