JPA - One-To-One associations explained

In JPA (Java Persistence API), a one-to-one association is a type of relationship between two entities where each instance of one entity is associated with exactly one instance of another entity. This relationship is represented using the @OneToOne annotation.

Key Points to Understand

1. Cardinality:
   • In a one-to-one relationship, each entity has a single corresponding entity. For example, if you have a Person entity and a Passport entity, each Person can have only one Passport, and each Passport can be assigned to only one Person.
2. Owning vs. Non-Owning Side:
   • Owning Side: The entity that contains the foreign key is considered the owning side of the relationship. This entity’s table will have the column that refers to the primary key of the other entity.
   • Non-Owning (Inverse) Side: The other entity that does not contain the foreign key. This entity maps the relationship using the mappedBy attribute in the @OneToOne annotation.
3. Bidirectional and Unidirectional Relationships:
   • Unidirectional: Only one entity is aware of the relationship. This means only one entity has the @OneToOne annotation, and the other entity has no reference to the relationship.
   • Bidirectional: Both entities are aware of the relationship. Each entity will have a reference to the other entity, and both will use the @OneToOne annotation.
4. Cascade Type:
   • Cascade operations can be applied to propagate operations (like persist, remove) from one entity to another. This is often used in one-to-one relationships to manage both entities together.
5. Fetch Type:
   • By default, one-to-one relationships use FetchType.EAGER, meaning the related entity is fetched immediately along with the owner entity. However, you can change this to FetchType.LAZY if you want to load the related entity on demand.

Example

Let’s consider an example where we have two entities: Person and Passport.

We wille be using the @MapsId annotation, where a Person entity has a one-to-one relationship with a Passport entity.

Explanation of @MapsId

• The @MapsId annotation maps the primary key of the child (Passport) to the primary key of the parent (Person).
• This enforces that both entities share the same primary key.
• The Passport entity doesn’t have its own generated ID but instead inherits the ID from Person.

Here’s the same JPA One-to-One relationship using @MapsId, but now with Lombok to simplify the entities. This reduces boilerplate code by automatically generating constructors, getters, setters, toString(), equals(), and hashCode().

Person.java

import jakarta.persistence.*;
import lombok.*;

@Entity
@Table(name = "person")
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString(exclude = "passport") // Avoid circular reference in toString()
public class Person {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    @OneToOne(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true)
    private Passport passport;

    public Person(String name) {
        this.name = name;
    }

    public void setPassport(Passport passport) {
        this.passport = passport;
        passport.setPerson(this); // Maintain bidirectional relationship
    }
}

Explanation

1. JPA Annotations
   • @Entity: Marks this class as a JPA entity.
   • @Table(name = "person"): Maps this entity to the person table in the database.
   • @Id: Marks the id field as the primary key.
   • @GeneratedValue(strategy = GenerationType.IDENTITY):
     • Uses the database’s identity column for automatic primary key generation.
     • Suitable for databases like MySQL and PostgreSQL.
   • @OneToOne(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true):
     • Declares a one-to-one relationship with Passport.
     • mappedBy = "person": The person field in Passport owns the relationship.
     • cascade = CascadeType.ALL: Any operations (persist, remove, merge) on Person cascade to Passport.
     • orphanRemoval = true: If a Person loses its Passport, the Passport is automatically deleted.
2. Lombok Annotations
   • @Getter @Setter: Automatically generates getter and setter methods.
   • @NoArgsConstructor: Generates a default constructor.
   • @AllArgsConstructor: Generates a constructor with all fields.
   • @ToString(exclude = "passport"): Prevents infinite recursion in toString() (since Person and Passport reference each other).
3. Custom setPassport() Method
   • Ensures both entities maintain a bidirectional relationship.
   • passport.setPerson(this); ensures that the Passport correctly references its Person.

---

Passport.java

import jakarta.persistence.*;
import lombok.*;

@Entity
@Table(name = "passport")
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@ToString(exclude = "person") // Avoid circular reference in toString()
public class Passport {

    @Id
    private Long id; // Uses the same ID as the Person entity

    private String passportNumber;

    @OneToOne
    @MapsId // Uses the same primary key as Person
    @JoinColumn(name = "id")
    private Person person;

    public Passport(String passportNumber) {
        this.passportNumber = passportNumber;
    }
}

Explanation

1. JPA Annotations
   • @Entity: Marks Passport as a JPA entity.
   • @Table(name = "passport"): Maps to the passport table.
   • @Id private Long id;: Uses the same primary key as Person.
   • @OneToOne:
     • Defines a one-to-one relationship with Person.
   • @MapsId: Maps id to the primary key of Person.
     • Passport does not have its own ID.
     • Instead, it shares the same ID as its Person.
   • @JoinColumn(name = "id"):
     • Specifies that the id column links to the primary key of Person.
2. Lombok Annotations
   • @Getter @Setter: Automatically generates getter and setter methods.
   • @NoArgsConstructor: Generates a default constructor.
   • @AllArgsConstructor: Generates a constructor with all fields.
   • @ToString(exclude = "person"): Prevents infinite recursion in toString().

---

3. Creating and Persisting Data

Main.java

public class Main {
    public static void main(String[] args) {
      EntityManagerFactory emf = HibernateConfig.createEntityManagerFactory();
        EntityManager em = emf.createEntityManager();
        EntityTransaction tx = em.getTransaction();

        try {
            tx.begin();

            // Create Person
            Person person = new Person("John Doe");

            // Create Passport
            Passport passport = new Passport("ABC123456");

            // Establish relationship
            person.setPassport(passport);

            // Persist Person (Passport will be automatically persisted due to CascadeType.ALL)
            em.persist(person);

            tx.commit();

            // Fetch and print data
            Person retrievedPerson = em.find(Person.class, person.getId());
            System.out.println("Person: " + retrievedPerson);
            System.out.println("Passport Number: " + retrievedPerson.getPassport().getPassportNumber());

        } catch (Exception e) {
            if (tx != null && tx.isActive()) {
                tx.rollback();
            }
            e.printStackTrace();
        } finally {
            em.close();
            emf.close();
        }
    }
}

Explanation

1. EntityManager Setup
   • EntityManagerFactory emf = HibernateConfig.createEntityManagerFactory();
   • EntityManager em = emf.createEntityManager();
   • Manages database operations.
2. Transaction Handling
   • tx.begin();: Starts a new transaction.
   • tx.commit();: Commits changes to the database.
   • If an exception occurs, tx.rollback(); is executed.
3. Creating and Persisting Entities
   • Creates a Person named "John Doe".
   • Creates a Passport with number "ABC123456".
   • Calls person.setPassport(passport); to establish the relationship.
   • Calls em.persist(person); to save Person (automatically persists Passport due to CascadeType.ALL).
4. Retrieving and Printing Data
   • Retrieves the Person from the database.
   • Prints Person and their associated Passport number.

---

4. Database Schema (Generated by Hibernate)

The generated schema will look like this:

Person Table

id	name
1	John Doe

Passport Table

id	passport_number
1	ABC123456

• The id of passport is the same as the id of person (due to @MapsId).
• This ensures a One-to-One relationship with shared primary keys.

---

Key Takeaways

1. One-to-One Relationship:
   • @OneToOne(mappedBy = "person") in Person.java.
   • @OneToOne @MapsId in Passport.java to share the same primary key.
2. Cascade & Orphan Removal:
   • CascadeType.ALL: Automatically saves/deletes the associated Passport when Person is modified.
   • orphanRemoval = true: If a Person is deleted or loses their Passport, the Passport is also deleted.
3. Lombok Simplifications:
   • @Getter, @Setter, @NoArgsConstructor, @AllArgsConstructor, and `@ToString