JPA Fundamentals & Hibernate - 5) Composed Keys

In this article, we are going to find out how to create composite keys in JPA.

Composite key is the key which composed of multiple columns.

Github Link

If you only need to see the code, here is the github link

Let’s say we have table called departmant and it has composed primary keys (for column code and number).

SQL Setup

Run the following sql query:

CREATE TABLE department
(
    code VARCHAR(100),
    number INT,
    name VARCHAR(100),
    PRIMARY KEY(code, number)
);

Create entities

We have options to create composite key:

Create Composed Keys with `@IdClass`

In the entity class, annotate all the primary keys field with @Id
Create a class which represents the composed key
- This class must have the same fields name as in the entity class
- Also this class must implements Serializable interface (JPA specification)
Annotate entity class with @IdClass(YourComposedClass.class)

Annotate all the primary keys field with `@Id`

@Entity
@Table(name = "department")
public class Department {
    @Id
    private String code;
    @Id
    private int number;
    private String name;

    // getters and setters..
}

Create a Composed key class

Fields in the composed key class must match with the fields annotated with @Id in the entity class

public class DepartmentPK implements Serializable {
    private String code;
    private int number;
    // getters and setters
}

Annotate entity class with `@IdClass`

@IdClass(DepartmentPK.class)
public class Department {}

Let’s create an department entity and persist it:

public class Main {

    public static void main(String[] args) {
        Department department = new Department();
        department.setNumber(10);
        department.setCode("AB");
        department.setName("test");

        try {
            entityManager.getTransaction().begin();
            entityManager.persist(department);
            entityManager.getTransaction().commit();
        }catch (Exception e) {
            System.out.println("Exception: " + e.getMessage());
        } finally {
            entityManager.close();
        }
    }
}

Then run the select query:

select * from department ;
 code | number | name
------+--------+------
 AB   |     10 | test
(1 row)

Create Composed Keys with Embeddable

Almost the same procedure as the @Embeddable /@Embedded in the previous blog. Only difference is that we should use @EmbeddedId and embeddable object must implements Serializable interface

First, create a new table called department_two

CREATE TABLE department_two
(
    code VARCHAR(100),
    number INT,
    name VARCHAR(100),
    PRIMARY KEY(code, number)
);

Next create embeddable composed key object:

@Embeddable
public class DepartmentEmbeddablePK implements Serializable {
    private int number;
    private String code;
    // getters and setters
}

Create an entity class:

@Entity
@Table(name = "department_two")
public class DepartmentTwo {

    @EmbeddedId
    @AttributeOverrides({
            @AttributeOverride(name = "number", column = @Column(name = "number")),
            @AttributeOverride(name = "code", column = @Column(name = "code")),
    })
    private DepartmentEmbeddablePK departmentEmbeddablePK;
    private String name;
    // getters and setters
}

Finally create an entity object and persist it:

public class Main {
    public static void main(String[] args) {
        DepartmentEmbeddablePK departmentEmbeddablePK = new DepartmentEmbeddablePK();
        departmentEmbeddablePK.setNumber(11);
        departmentEmbeddablePK.setCode("BC");

        DepartmentTwo departmentTwo = new DepartmentTwo();
        departmentTwo.setName("test2");
        departmentTwo.setDepartmentEmbeddablePK(departmentEmbeddablePK);

        try {
            entityManager.getTransaction().begin();
            entityManager.persist(departmentTwo);
            entityManager.getTransaction().commit();
        }catch (Exception e) {
            System.out.println("Exception: " + e.getMessage());
        } finally {
            entityManager.close();
        }
    }
}