The basic mapping concepts

When learning Hibernate, many like to jump to Parent – Child associations without mastering the object relation mapping basics. It’s very important to understand the basic mapping rules for individual Entities before starting modelling Entity associations.

Hibernate types

A Hibernate type is a bridge between an SQL type and a Java primitive/Object type.

These are the types Hibernate supports by default:

You can always define your own custom types as we will see in a future article.

Embedded (a.k.a Component) Types

You can group multiple columns to a specific Java type that can be reused throughout your domain model. If the mapped Java object is always dependent on some external Entity you can choose an Embeddable type for such domain model mapping.

An Embeddable object may contain both basic types and association mappings but it can never contain an @Id. The Embeddable object is persisted/removed along with its owning entity.

Assuming we have the following SQL table:

CREATE TABLE entity_event
  (
     id           BIGINT GENERATED BY DEFAULT 
                  AS IDENTITY (START WITH 1),
     entity_class VARCHAR(255),
     entity_id    BIGINT,
     message      VARCHAR(255),
     PRIMARY KEY (id)
  );  

We could group the entity_class and entity_id to an Embeddable object that we’ll employ in two different owning Entities.

The Embeddable object looks like this:

@Embeddable
public class EntityIdentifier 
    implements Serializable {

    @Column(name = "entity_id", 
         nullable = true)
    private Long entityId;

    @Column(name = "entity_class", 
         nullable = true)
    private Class entityClass;

    public EntityIdentifier() {
    }

    public EntityIdentifier(Class entityClass, 
        Long entityId) {
        this.entityClass = entityClass;
        this.entityId = entityId;
    }

    public Class getEntityClass() { 
        return entityClass; 
    }

    public void setEntityClass(Class entityClass) { 
        this.entityClass = entityClass; 
    }

    public Long getEntityId() { 
        return entityId; 
    }

    public void setEntityId(Long entityId) { 
        this.entityId = entityId; 
    }
}

The associated Entity table will inherit the Embeddable properties associated columns.

Entity

An Entity is the Java equivalent of an SQL table row. The entity must contain an @Id property mapping the associated table Primary Key.

The application logic makes changes to Entities properties and notifies the Persistence Context of Entity state changes (persist, merge, remove). The persistence context will therefore translate all Entity changes to SQL statements.

Assuming we have the following SQL tables:

CREATE TABLE entity_attribute
  (
     id           BIGINT GENERATED BY DEFAULT 
                  AS IDENTITY (START WITH 1),
     entity_class VARCHAR(255),
     entity_id    BIGINT,
     name         VARCHAR(255),
     VALUE        VARCHAR(255),
     PRIMARY KEY (id)
  );
 CREATE TABLE entity_event
  (
     id           BIGINT GENERATED BY DEFAULT 
                  AS IDENTITY (START WITH 1),
     entity_class VARCHAR(255),
     entity_id    BIGINT,
     message      VARCHAR(255),
     PRIMARY KEY (id)
  );    

We can make use of the EntityIdentifier Embeddable type since both tables contain the entity_class and entity_id columns.

@Entity
@Table(name = "entity_attribute")
public class EntityAttribute {

    @Id
    @GeneratedValue
    private Long id;

    private String name;

    private String value;

    private EntityIdentifier entityIdentifier;

    public Long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getValue() {
        return value;
    }

    public void setValue(String value) {
        this.value = value;
    }

    public EntityIdentifier getEntityIdentifier() {
        return entityIdentifier;
    }

    public void setEntityIdentifier(
        EntityIdentifier entityIdentifier) {
        this.entityIdentifier = entityIdentifier;
    }
}

@Entity
@Table(name = "entity_event")
public class EntityEvent {

    @Id
    @GeneratedValue
    private Long id;

    private String message;

    private EntityIdentifier entityIdentifier;

    public Long getId() {
        return id;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public EntityIdentifier getEntityIdentifier() {
        return entityIdentifier;
    }

    public void setEntityIdentifier(
        EntityIdentifier entityIdentifier) {
        this.entityIdentifier = entityIdentifier;
    }
}

Testing time

We will create one EntityEvent and one EntityAttribute for a given Product to see how the Embeddable is being persisted along with the owning entities:

@Test
public void testEntityIdentifier() {
    doInTransaction(session -> {
        Product product = new Product("LCD");
        session.persist(product);
        EntityEvent productEvent = new EntityEvent();
        productEvent.setMessage(String.format(
            "Product %s added", product.getName()));
        productEvent.setEntityIdentifier(
            new EntityIdentifier(
                product.getClass(),
                product.getId()
        ));
        session.persist(productEvent);
        EntityAttribute productAttribute = 
            new EntityAttribute();
        productAttribute.setName("AD_CAMPAIGN");
        productAttribute.setValue("LCD_Sales");
        productAttribute.setEntityIdentifier(
            new EntityIdentifier(
                product.getClass(),
                product.getId()
        ));
        session.persist(productAttribute);
        assertSame(1, session.createQuery(
            "select ea " +
            "from EntityAttribute ea " +
            "where " +
            "   ea.entityIdentifier = :entityIdentifier")
            .setParameter("entityIdentifier",
                new EntityIdentifier(
                    product.getClass(), product.getId()))
            .list().size());
        return null;
    });
}

Running this test generates the following SQL output:

INSERT INTO product (
    id, 
    name
)
VALUES (
    DEFAULT, 
    'LCD'
);

INSERT INTO entity_event (
    id,
    entity_class,
    entity_id,
    message
)
VALUES (
    DEFAULT,
    'com.vladmihalcea.hibernate.masterclass.laboratory.entityidentifier.Product',
    1,
    'Product LCD added'
);

INSERT INTO entity_attribute (
    id,
    entity_class,
    entity_id,
    name,
    VALUE
)
VALUES (
    DEFAULT,
    'com.vladmihalcea.hibernate.masterclass.laboratory.entityidentifier.Product',
    1,
    'AD_CAMPAIGN',
    'LCD_Sales'
);

SELECT entityattr0_.id           AS id1_0_,
       entityattr0_.entity_class AS entity_c2_0_,
       entityattr0_.entity_id    AS entity_i3_0_,
       entityattr0_.name         AS name4_0_,
       entityattr0_.VALUE        AS value5_0_
FROM   entity_attribute entityattr0_
WHERE  entityattr0_.entity_class = 'com.vladmihalcea.hibernate.masterclass.laboratory.entityidentifier.Product'
       AND entityattr0_.entity_id = 1;

I'm running an online workshop on the 20-21 and 23-24 of November about High-Performance Java Persistence.

If you enjoyed this article, I bet you are going to love my Book and Video Courses as well.

Conclusion

There are still many concepts we need to cover before getting to understand Entity associations. You should always take you time to understand the basic concepts before jumping to more advanced topics. My next post will be about Entity Identifiers and all the available generator techniques.

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