Hibernate @OneToOne with superclass, only retrieve superclass fields on join

Question

I have mapped my inheritance hierarchy in Hibernate using InheritanceType.Single_Table and discriminator columns to distinguish between the different entities. All subclasses of the superclass store their fields into secondary tables. As an example:

@MappedSuperclass
public abstract class Base
{
   @Id 
   private String id;

   @Version
   private long version;
}

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.STRING) 
public class Parent extends Base
{
    @Column(nullable=false)
    private BigDecimal value;
}

@Entity
@DiscriminatorValue("child1")
@SecondaryTable(name = "Child1")
public class Child1 extends Parent
{
     @Column(table="Child1")
     private String name;
}

@Entity
@DiscriminatorValue("child2")
@SecondaryTable(name = "Child2")
public class Child2 extends Parent
{
     @Column(table="Child2")
     private String name2;
} 

I now have an Entity that has a @OneToOne relationship with the Parent class. This Entity only needs to work with the value field from the Parent class. It will never need to work with any fields from any subclass of Parent

@Entity
public class AnotherEntity extends Base
{
    @JoinColumn(name="parentId")
    @OneToOne(fetch=FetchType.Lazy, optional=true, targetEntity=Parent.class)
    private Parent parent;
 }

What I want to happen is that only the fields of Parent.class are selected when the relationship to parent is loaded from the database. What I'm seeing is that Hibernate attempts to load all properties of the entities that extend Parent. It also left joins all of the Secondary tables. This is problematic as I have rougly 30 entities that extend Parent. This makes fetching the Parent entity non-viable as the query performs 30 joins.

As an example, this is the type of query I am seeing:

Hibernate: 
select
    parent.id as id3_0_,
    parent_.version as version3_0_,
    parent.name1 as name110_3_0_,
    parent.name2 as name24_3_0_,
    parent.type as type3_0_ 
from
    Parent parent0_ 
left outer join
    Child1 parent0_2_ 
        on parent0_.id=parent0_2_.id 
left outer join
    Child2 parent0_3_ 
        on parent0_.id=parent0_3_.id 

I don't understand why Hibernate decides to select a superset of all properties defined in the subclasses of Parent and join all of the secondary tables? I could understand it joining the secondary table for entity defined by the discriminator value of the parent being referenced, but otherwise I am confused.

My question is, how do I go about achieving my requirement of only having the fields from the Parent class loaded when I retrieve the Parent relationship in the AnotherEntity class?

Thanks.

Answer 1

A secondary table is normally used to map the content of a single entity to two tables. It doesn't allow for lazy/select fetching using standard JPA annotations. You may use a proprietary Hibernate annotation to load it using a separate select, and only if necessary, though. See http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#mapping-declaration-join :

fetch: If set to JOIN, the default, Hibernate will use an inner join to retrieve a secondary table defined by a class or its superclasses and an outer join for a secondary table defined by a subclass. If set to SELECT then Hibernate will use a sequential select for a secondary table defined on a subclass, which will be issued only if a row turns out to represent an instance of the subclass. Inner joins will still be used to retrieve a secondary defined by the class and its superclasses.

So setting the fetch attribute of the Hibernate @Table annotation to SELECT will do what you want : an additional select clause will be issued to select the values from just the appropriate secondary table.

If you want lazy fetching, then a secondary table is not what you want. You'll have to do it using associations.