JPA 实体与外键的多对一双向关系的最佳方式

Question

Considering the below table structure and relation.

Teacher
id(pk),
tname,
subject_id(fk),
subject_name

Subject
sid(pk),
sname,
subject_learning_curve

TEACHER(owner of relationship) to SUBJECT Many to One, bidirectional relationship

I used persistence tool window here in intellij. Which based on the data source generates the Entities automatically for all the relationships in DB.

In the entity it is generated like this

@Entity
//using lombok for getters and setters
public class TEACHER{
     
    @Id
    @GeneratedValue
    private Long id;
    private String tname;
    private long subject_id;
    private String subject_name;

    @ManyToOne
    @JoinColumn(name = "subject_id",referencedColumnName="sid")
    private Subject subjectBySid;    //is this fine to have a reference like this
}

OR 2nd approach below

@Entity
    //using lombok for getters and setters
    public class Teacher{
         
        @Id
        @GeneratedValue
        private Long id;
        private String tname;
        @ManyToOne
        @JoinColumn(name = "subject_id",referencedColumnName="sid")
        private Subject subject;    //or just this it will suffice all use case,like if i just want to read Teacher data and just the sid not the whole subject row
    }

Similarly Subject will also have reference pointing back to Teacher table.

Based on below points,can someone provide insights.

1. Which is ideal and correct out of the above two ways of writing a entity.
2. From Hibernate and JPA perspective which is more efficient.
3. Will there be any difference on the sql generated based on what approach i chose to write my entity.
4. If i take 2nd approach will it be more easier to query and save records to DB or to retrieve.
5. Would the subject_id in TEACHER have value when i get the data in Entity or it will be just a reference named subject,and i would not get that specific field in TEACHER.subject_id or @JoinColumn(name = "subject_id",referencedColumnName="sid") annotation will take care of that and it should have value in TEACHER.subject_id

Answer 1

Below are my answers and suggestions on your questions:

1.Which is ideal and correct out of the above two ways of writing a entity.

Approach 2 is correct way of writing an enitiy. Once you have defined @ManyToOne relation with another entity,no need to define columns specific to that entity.

2. From Hibernate and JPA perspective which is more efficient.

JPA is the specification for management of relational data in Java and Hibernate is the Object-Relational Mapping(ORM) tool that saves the state of objects into DB. Simply put, Hibernate is the implementation of JPA. Hence, comparing them should not be a question. You can compare Hibernate with other ORM tools that can be used with Spring framework like iBatis, EclipseLink,etc.

3. Will there be any difference on the sql generated based on what approach i chose to write my entity.

No,generated SQL will be the same. But, to generate that SQL, you can use different approaches like @NativeSqlQuery , Hibernate Query Language(HQL) ,etc.

4.If i take 2nd approach will it be more easier to query and save records to DB or to retrieve.

You should definitely use 2nd approach for clear,staright-forward and relational approach. 1st approach is wrong to its core if you take about the principles of object-oriented language and relational database management. No need to create extra columns,code,entity properties, and other mistakes when you can easily link them through mapping.

5.Would the subject_id in TEACHER have value when i get the data in Entity or it will be just a reference named subject,and i would not get that specific field in TEACHER.subject_id or @JoinColumn(name = "subject_id",referencedColumnName="sid") annotation will take care of that and it should have value in TEACHER.subject_id

In your DB,you should have a column subject_id in your Teacher table that will be a foreign key and references sid column of Subject table. This happens at DB end.
At your Object-oriented end (Spring), you can use create object reference for Subject to map with Student or you can directly use field subject_id in your HQL or NativeQuery to perform DB operations and persist the data. Hence, you will get subject_id in your Teacher entity.