Java.Hibernate.Beginner.How do you implement a composite key in Hibernate?

A composite key means the primary key of a table is made up of two or more columns, and you need to map this combined key in your entity.
In Hibernate (and JPA), you implement composite keys using either:

• @Embeddable + @EmbeddedId → recommended, modern, type-safe approach.
• @IdClass → older, more verbose alternative.

🔹 Approach 1: @Embeddable + @EmbeddedId (recommended)

✅ Step 1: Create a key class annotated with @Embeddable

@Embeddable
public class EnrollmentId implements Serializable {
    private Long studentId;
    private Long courseId;

    // Must have default constructor, getters/setters, equals, and hashCode!
    public EnrollmentId() {}
    // getters, setters, equals, hashCode...
}

✅ Step 2: Use @EmbeddedId in your entity

@Entity
public class Enrollment {
    @EmbeddedId
    private EnrollmentId id;

    @ManyToOne
    @MapsId("studentId") // maps studentId in EnrollmentId
    @JoinColumn(name = "student_id")
    private Student student;

    @ManyToOne
    @MapsId("courseId") // maps courseId in EnrollmentId
    @JoinColumn(name = "course_id")
    private Course course;

    private String grade;

    // getters and setters
}

🔹 What happens:

• The table enrollment has a composite primary key of (student_id, course_id).
• Hibernate maps these two columns together as EnrollmentId.

🔹 Generated SQL table:

CREATE TABLE enrollment (
  student_id BIGINT NOT NULL,
  course_id BIGINT NOT NULL,
  grade VARCHAR(10),
  PRIMARY KEY (student_id, course_id),
  FOREIGN KEY (student_id) REFERENCES student(id),
  FOREIGN KEY (course_id) REFERENCES course(id)
);

🔹 Approach 2: Using @IdClass
✅ Define a simple class with the same fields as the composite key:

public class EnrollmentId implements Serializable {
    private Long studentId;
    private Long courseId;
    // default constructor, equals, hashCode...
}

@Entity
@IdClass(EnrollmentId.class)
public class Enrollment {
    @Id
    private Long studentId;

    @Id
    private Long courseId;

    private String grade;
    // getters and setters
}

🔹 Key difference:

• @IdClass keeps key fields in the entity itself, while @EmbeddedId uses a separate embedded object.

When to use which?
✅ Use @EmbeddedId for cleaner, more object-oriented design → easier to reuse key classes, especially with associations.
✅ Use @IdClass if you want key fields directly in your entity.

✅ Key takeaway:
To implement composite keys in Hibernate:

• Create an @Embeddable class representing the key.
• Use @EmbeddedId in your entity.
• Make sure your key class has proper equals() and hashCode() implementations!