如何在同一个数据库表上映射两个 JPA 或 Hibernate 实体

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 .

Answer 2

I have a similar Problem. I have two classes, the base class DocumentContent and a derived class DocumentContentWithData :

DocumentContent {
    private Long documentContentId;
    private InputStream contentStream;
    private File importFile;
    ... and several other attributes
}

DocumentContentWithData extends DocumentContent {}

I defined a hibernate mapping for the two classes, for the class DocumentContent a mapping without the contentStream and for the class DocumentContentWithData a mapping with the contentStream .

Saving the data with the class DocumentContentWithData works and also getting the instances for both classes works with the method Session.get(Class, ID) .

However the query in the following code returns two entities, one as instance of DocumentContent and the other as instance of DocumentContentWithData although there is only one entry with the importFile in the database:

CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();
CriteriaQuery<DocumentContent> criteriaQuery = criteriaBuilder.createQuery(DocumentContent.class);
Root<DocumentContent> root = criteriaQuery.from(DocumentContent.class);
criteriaQuery.select(root).where(criteriaBuilder.equal(root.get("importFile"), importFile));
Query<DocumentContent> query = session.createQuery(criteriaQuery);
List<DocumentContent> contentList = query.getResultList();

Can someone explain this behavior. When I do not derive the class DocumentContentWithData from DocumentContent and duplicate all attributes it works. However this is not a very nice solution.

Answer 3

You have to add annotation @Entity,@Table(name="RESTAURANT") on your class, add annotations and replace your detailed mapping in hbm file by
.

Here a complete example: http://viralpatel.net/blogs/hibernate-many-to-many-annotation-mapping-tutorial/