【Spring Data JPA】Join Table Entity

Composite key

@Entity
public class BookAuthor {
    @EmbeddedId
    private BookAuthorId id;

    @ManyToOne
    @MapsId("bookId")
    private Book book;

    @ManyToOne
    @MapsId("authorId")
    private Author author;

    // Other fields, getters, setters...
}

@Embeddable
public class BookAuthorId implements Serializable {
    private Long bookId;
    private Long authorId;

    // Constructors, equals, hashCode...
}

Pros

Naturally represents the relationship between entities.
Ensures data integrity at the database level.
Can be more space-efficient in some cases.
Useful when the combination of fields has a natural, real-world uniqueness.

Cons

Can be more complex to work with in queries and code.
May lead to longer primary keys, which can impact performance in large tables.
Less flexible if the relationship structure changes.

Separate primary key

@Entity
public class BookAuthor {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    private Book book;

    @ManyToOne
    private Author author;

    // Other fields, getters, setters...
}

Pros

Simpler to work with in code and queries.
More flexible if relationships change.
Can improve performance in some scenarios, especially with large datasets.
Easier to reference from other tables.

Cons

Requires an additional column in the table.
May not represent the natural relationship as clearly.

Consideration

Consider using a Composite Key

The combination of foreign keys naturally and uniquely identifies the relationship.
You want to enforce referential integrity at the database level.
The relationship is stable and unlikely to change.
You’re working with a relatively small dataset.

Consider using a Separate Primary Key

1 You need more flexibility in your data model.
2 You’re working with large datasets where query performance is crucial.
3 You anticipate changes in the relationship structure.
4 You want to simplify your code and queries.
5 You need to reference this join table from other entities.

Additional Considerations

Database Performance: Test both approaches with your expected data volume to see which performs better.
ORM Tool: Some ORM tools work better with one approach over the other.
Team Preference: Consider what your team is more comfortable working with.
Existing Patterns: If your project already uses one approach consistently, it might be best to stick with it for consistency.

In many modern applications, using a separate primary key is often preferred due to its simplicity and flexibility. However, there are still valid use cases for composite keys, especially in systems where the relationship itself has significant business meaning.