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Composite Key with JPA and Hibernate

Composite Key with JPA and Hibernate

In this article let discuss on

How to map Composite Key  with JPA and Hibernate

Domain Model

A relational database composite key contains two or more columns which together for the primary key of a given table.

Composite Key with JPA and Hibernate

In the diagram above, the employee table has a Composite Key, which consists of two columns:

    • company_id
    • employee_number

Every Employee can also have a Phone, which uses the same composite key to reference its owning Employee.

Composite Primary Key with JPA and Hibernate

To map this database table mapping, we need to isolate the compound key into an @Embeddable first:

@Embeddable
public class EmployeeId implements Serializable {
 
    @Column(name = "company_id")
    private Long companyId;
 
    @Column(name = "employee_number")
    private Long employeeNumber;
 
    public EmployeeId() {
    }
 
    public EmployeeId(Long companyId, Long employeeId) {
        this.companyId = companyId;
        this.employeeNumber = employeeId;
    }
 
    public Long getCompanyId() {
        return companyId;
    }
 
    public Long getEmployeeNumber() {
        return employeeNumber;
    }
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof EmployeeId)) return false;
        EmployeeId that = (EmployeeId) o;
        return Objects.equals(getCompanyId(), that.getCompanyId()) &&
                Objects.equals(getEmployeeNumber(), that.getEmployeeNumber());
    }
 
    @Override
    public int hashCode() {
        return Objects.hash(getCompanyId(), getEmployeeNumber());
    }
}

The JPA specification says that all entity identifiers should be serializable and implement equals and hashCode.

So, an Embeddable that is used as a composite identifier must be Serializable and implement equals and hashCode.

The Employee mapping looks as follows:

@Entity(name = "Employee")
@Table(name = "employee")
public class Employee {
 
    @EmbeddedId
    private EmployeeId id;
 
    private String name;
 
    public EmployeeId getId() {
        return id;
    }
 
    public void setId(EmployeeId id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}


The @EmbeddedId is used to instruct Hibernate that theEmployeeentity uses a compound key.

The Phone mapping is rather straightforward as well:

@Entity(name = "Phone")
@Table(name = "phone")
public class Phone {
 
    @Id
    @Column(name = "`number`")
    private String number;
 
    @ManyToOne
    @JoinColumns({
        @JoinColumn(
            name = "company_id",
            referencedColumnName = "company_id"),
        @JoinColumn(
            name = "employee_number",
            referencedColumnName = "employee_number")
    })
    private Employee employee;
 
    public Employee getEmployee() {
        return employee;
    }
 
    public void setEmployee(Employee employee) {
        this.employee = employee;
    }
 
    public String getNumber() {
        return number;
    }
 
    public void setNumber(String number) {
        this.number = number;
    }
}

The Phone uses the number as an entity identifier since every phone number and the @ManyToOne mapping uses the two columns that are part of the compound key.

Mapping relationships using the Composite Key

We can even map relationships using the information provided within the Composite Key itself. In this particular example, the company_id references a Company entity which looks as follows:

@Entity(name = "Company")
@Table(name = "company")
public class Company {
 
    @Id
    private Long id;
 
    private String name;
 
    public Long getId() {
        return id;
    }
 
    public void setId(Long id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Company)) return false;
        Company company = (Company) o;
        return Objects.equals(getName(), company.getName());
    }
 
    @Override
    public int hashCode() {
        return Objects.hash(getName());
    }
}

We can have the Composite Key mapping referencing the Company entity within the Employee entity:

@Entity(name = "Employee")
@Table(name = "employee")
public class Employee {
 
    @EmbeddedId
    private EmployeeId id;
 
    private String name;
 
    @ManyToOne
    @JoinColumn(name = "company_id",insertable = false, updatable = false)
    private Company company;
 
    public EmployeeId getId() {
        return id;
    }
 
    public void setId(EmployeeId id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}

Notice that the @ManyToOne association instructs Hibernate to ignore inserts and updates issued on this mapping since the company_idis controlled by the @EmbeddedId.

 

Mapping a relationships inside @Embeddable

But that’s not all. We can even move the @ManyToOne inside the @Embeddable itself:

@Embeddable
public class EmployeeId implements Serializable {
 
    @ManyToOne
    @JoinColumn(name = "company_id")
    private Company company;
 
    @Column(name = "employee_number")
    private Long employeeNumber;
 
    public EmployeeId() {
    }
 
    public EmployeeId(Company company, Long employeeId) {
        this.company = company;
        this.employeeNumber = employeeId;
    }
 
    public Company getCompany() {
        return company;
    }
 
    public Long getEmployeeNumber() {
        return employeeNumber;
    }
 
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof EmployeeId)) return false;
        EmployeeId that = (EmployeeId) o;
        return Objects.equals(getCompany(), that.getCompany()) &&
                Objects.equals(getEmployeeNumber(), that.getEmployeeNumber());
    }
 
    @Override
    public int hashCode() {
        return Objects.hash(getCompany(), getEmployeeNumber());
    }
}

Now, the Employee mapping will no longer require the extra @ManyToOne association since it’s offered by the entity identifier:

@Entity(name = "Employee")
@Table(name = "employee")
public class Employee {
 
    @EmbeddedId
    private EmployeeId id;
 
    private String name;
 
    public EmployeeId getId() {
        return id;
    }
 
    public void setId(EmployeeId id) {
        this.id = id;
    }
 
    public String getName() {
        return name;
    }
 
    public void setName(String name) {
        this.name = name;
    }
}

The persistence logic changes as follows:

Company company = doInJPA(entityManager -> {
    Company _company = new Company();
    _company.setId(1L);
    _company.setName("vladmihalcea.com");
    entityManager.persist(_company);
    return _company;
});
 
doInJPA(entityManager -> {
    Employee employee = new Employee();
    employee.setId(new EmployeeId(company, 100L));
    employee.setName("Vlad Mihalcea");
    entityManager.persist(employee);
});
 
doInJPA(entityManager -> {
    Employee employee = entityManager.find(
        Employee.class,
        new EmployeeId(company, 100L)
    );
    Phone phone = new Phone();
    phone.setEmployee(employee);
    phone.setNumber("012-345-6789");
    entityManager.persist(phone);
});
 
doInJPA(entityManager -> {
    Phone phone = entityManager.find(Phone.class, "012-345-6789");
    assertNotNull(phone);
    assertEquals(new EmployeeId(company, 100L), phone.getEmployee().getId());
});

Conclusion

Knowing how to map a Composite Key with JPA and Hibernate is very important because this is the way you’d map a many-to-many association.

As demonstrated in this blog post, such a mapping is not complicated at all.

Balamurugan T

Balamurugan T

August 05, 2021

Java & C#.NET Developer with 10++ years of IT experience.

Planning to learn ReactJS or Angular or Flutter.!

Difference between Entity vs Model

Difference between Entity vs Model

Difference between Entity vs Model

Entity: An entity represents a single instance of your domain object saved into the database as a record. It has some attributes that we represent as columns in our tables.

Model: A model typically represents a real world object that is related to the problem or domain space. In programming, we create classes to represent objects. These classes, known as models, have some properties and methods (defining objects behaviour).

ViewModel: The term ViewModel originates from the MVVM (Model View ViewModel) design pattern. There are instances in which the data to be rendered by the view comes from two different objects. In such scenarios, we create a model class which consists of all properties required by the view. It’s not a domain model but a ViewModel because, a specific view uses it. Also, it doesn’t represent a real world object.

DataModel: In order to solve a problem, objects interact with each other. Some objects share a relationship among them and consequently, form a data model that represents the objects and the relationship between them.

In an application managing customer orders, for instance, if we have a customer and order object then these objects share a many to many relationship between them. The data model is eventually dependent on the way our objects interact with each other. In a database, we see the data model as a network of tables referring to some other tables.

 

Pro Tip:

In any programming language always hide your entity object to the client. use model and mapper to share the object to the client.

Balamurugan T

Balamurugan T

August 05, 2021

Java & C#.NET Developer with 10++ years of IT experience.

Planning to learn ReactJS or Angular or Flutter.!