How to map two JPA or Hibernate entities on the same database table

Question

In our project, we have an entity 'Restaurant' with nearly 30 fields(some have relationships with other entities). So, every time we need a 'Restaurant' object even for a few fields, all the others are retrieved. This effects the performance. So, in the HBM file, we wrote two classes both pointing to the same physical class and same database table, as shown below.

=== restaurant.hbm.xml ===
<!-- Light Weight Version -->
<class name="com.raj.model.Restaurant" table="RESTAURANTS" entity-name="RestaurantLite" 
                dynamic-update="false" dynamic-insert="false">
<cache usage="read-only"/>
     <!-- few basic properties and relationships -->
</class>

<!-- Restaurant -->
<class name="com.raj.model.Restaurant" table="RESTAURANTS" entity-name="Restaurant">
     <!-- all properties and relationships -->
</class>

In one of the DAO implementations, we are using Criteria which takes 'RestaurantLite' and returning list of restaurants as shown below.

Criteria criteria = session.createCriteria("RestaurantLite");

   // criteria related stuff

return new LinkedHashSet<Restaurant>(criteria.list());

Now we want to remove all hbm files and use annotations. So how the same can be done using annotations for entites? Do we need to create an extra class 'RestaurantLite'? If then, how the above criteria returns 'Restaurant' objects??

Answer 1

To summarize it, the following mappings are going to demonstrate how you can map multiple entities to the same database table:

@Entity(name = "Post")
public class Post {

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

    private String name;

    private String description;

    public String getName() {
        return name;
    }

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

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }
}

@Entity(name = "PostSummary")
@Table(name = "Post")
@Immutable
public class PostSummary {

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

    private String name;

    public String getName() {
        return name;
    }

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

@Entity(name = "UpdatablePostSummary")
@Table(name = "Post")
@DynamicUpdate
public class UpdatablePostSummary {

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

    private String name;

    public String getName() {
        return name;
    }

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

And Hibernate will work just fine:

@Test
public void testOneTableMultipleEntities() {
    doInTransaction(session -> {
        Post post = (Post) session.get(Post.class, 1L);
        PostSummary postSummary = (PostSummary) session.get(PostSummary.class, 1L);
        UpdatablePostSummary updatablePostSummary = (UpdatablePostSummary) session.get(UpdatablePostSummary.class, 1L);
        assertEquals(post.getName(), postSummary.getName());
        assertEquals(post.getName(), updatablePostSummary.getName());
        updatablePostSummary.setName("Hibernate Master Class Tutorial.");
    });
}

1. The PostSummary is just a read-only View over your original entity, hence I annotated it with @Immutable.

2. The UpdatablePostSummary is marked with @DynamicUpdate and so you can propagate changes from this View entity too.

This test is also available on GitHub.