Problem loading Set of subclass (Table per Subclass)

paulf · **Joined:** Thu Jan 13, 2005 10:04 am **Posts:** 18

I have mapped an inheritance hierarchy using the 'Table per subclass' strategy for the classes Child and Parent (Child extends Parent). The subclass Child has a primary key association to the Parent superclass making it a one-to-one association. There is a third class that I am calling A. A has a bidirectional one-to-many relationship with Child.

Code:

@Entity
class A {
    Set<Child> children;

    @OneToMany(mappedBy="a") @IndexColumn(name="id")
    @JoinColumn(name="some_fk")
    public Set getChildren();
 }
 

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
abstract class Parent {
    A a;
    @ManyToOne
    @JoinColumn(name="some_fk", nullable=false)   
    public A getA();
 
 } 

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
@PrimaryKeyJoinColumn(name="id")
class Child extends Parent {
     ...  
 }

I can successfully save an instance of Child and get the expected values in the two relevant tables and I can query using an explicit 'from Child' or polymorphic query 'from Parent' and get the correct values back.

I can also save an instance of A that has added an instance of Child to the Set children and everything looks fine, I get the correct values saved and the foreign and primary keys are as expected.

If, however, I load and instance of A (which works) and try and access the Set children I get an exception:

Code:

ERROR org.hibernate.util.JDBCExceptionReporter - Unknown column 'child0_.some_fk' in 'field list'

It would appear that Hibernate is trying to load the Set of Child expecting to find the foreign key 'some_fk' in the Child table, I have this mapping mapped via the @ManyToOne annotation in the Parent class and hence in the Parent table.

Am I able to map such a relationship? If so, any ideas where I am going wrong!

paulf · **Joined:** Thu Jan 13, 2005 10:04 am **Posts:** 18

I found the solution, I added the propert targetEntity to the @OneToMany annotation:

Code:

@Entity
class A {
    Set<Child> children;

    @OneToMany(mappedBy="a", targetEntity=Parent.class)
    @IndexColumn(name="id")
    @JoinColumn(name="some_fk")
    public Set getChildren();
 } 

paulf · **Joined:** Thu Jan 13, 2005 10:04 am **Posts:** 18

Help!

Well I had thought I had solved this problem but it would seem I am not quite there.

When I call getChildren I noticed the following hibernate query was generated (I have changed the names of the classes used and removed the as value_0_, essentially I have 3 subclasses and the superclass mapped using the 'table-per-subclass':

Code:

select child0_.some_fk, child0_1_.value, child0_2_.value, child0_3_.value, 
case 
 when child0_1_.id is not null then 1 
 when child0_2_.id is not null then 2 
 when child0_3_.id is not null then 3 
 when child0_.id is not null then 0 
end as clazz_0_ from

Parent child0_ left outer join ChildType1 child0_1_ on child0_.id=child0_1_.id 
left outer join ChildType2 child0_2_ on child0_.id=child0_2_.id 
left outer join ChildType3 child0_3_ on child0_.id=child0_3_.id 
where child0_.some_fk=?

I want to ONLY get the ChildType1 back - I can't see why the case statement is necessary.

Also, I changed from Set<Child> to List<Child> as I would like to use the IndexColumn annotation as I believe this will help with deleting from the List - which I am unable to do. I can add to the List but when I try and remove an object from the List that was returned using the getChilldren method the only thing that updates is class A.

Some help would be very welcome. I have read and re-read the documentation and the forums. I am starting to think that perhaps mapping an inheritance hierarchy is more trouble than it's worth (that is unless I am doing something wrong)...

paulf · **Joined:** Thu Jan 13, 2005 10:04 am **Posts:** 18

Okay - well I am getting somewhere. To solve the delete problem that I had I added the following annotation to the List<ChildType1> getChildren method:

Code:

@org.hibernate.annotations.Cascade( { org.hibernate.annotations.CascadeType.ALL, org.hibernate.annotations.CascadeType.DELETE_ORPHAN })

and removed the cascade = CascadeType.ALL from the @OneToMany annotation.

I also removed the @IndexColumn as it was causing problems - I think I'll come back to this one later...

If anyone could help with why I get the rather complicated query (see previous post) when I call the getChildren method that would be great.

Is this expected behaviour?

riccardi · **Joined:** Tue May 30, 2006 8:45 pm **Posts:** 1

I believe that I have a similar problem to this one, although I am using a mapping file and not using annotations. I have not found sufficient information in this thread to isolate my problem.

I am successfully using many-to-one unidirectional associations in Hibernate 3 to access my database objects as Java objects. I am trying to extend the hibernate mappings to add the reverse associations, with little success.

Specifically, I am trying to build a bi-directional many-to-one association based on a foreign key between 2 subclasses of a common base class. When I try to traverse the one-to-many side, I get an exception dump which includes "Unknown column 'imagesx0_.specimenId' in 'field list'" The table name used here is based on the name of the <set > mapping, not of the database table.

I believe the problem is that hibernate is choosing the wrong name for the related table. When I change the name of the set mapping, the error message changes. In the above, I had changed the name of the Set from "images" to "imagesX" and the error changed from "Unknown column 'images0_.specimenId'..." to "Unknown column 'imagesx0_.specimenId' ...".

The actual table name is "image"

Some details of the code and mapping file follow. The many-to-one direction of this mapping works properly, but not the reverse.

I would appreciate any ideas about what I've done wrong in this system. I can supply more details, if needed.

I have a mapping file with a base class (BaseObject) and 2 subclasses Image and Specimen, and tables baseObject, image and specimen. The image table has a specimenId field that is a foreign key to specimen. Both image and specimen have an "id" field that is a foreign key to baseObject.

The Java classes include

Code:

class BaseObject { String id ...}
class Specimen extends BaseObject {... Set imagesX;...}
class Image extends BaseObject {...Specimen specimen;...}

code fragment that causes error is:

Code:

getImagesX().iterator

The mapping files include

Code:

   <subclass name="net.morphbank.rdfmetadata.Specimen"
      extends="net.morphbank.rdfmetadata.BaseObject"
      discriminator-value="Specimen">
      <set name="imagesX" table="image" inverse="true" >
         <key column="specimenId" />
         <one-to-many class="net.morphbank.rdfmetadata.Image" />
      </set>

      <join table="SpecimenExtended">

... and...

Code:

      <subclass name="net.morphbank.rdfmetadata.Image" extends="net.morphbank.rdfmetadata.BaseObject"
         discriminator-value="Image">
         <join table="image">
            <key column="id" />
               </property>
            <many-to-one name="specimen" class="net.morphbank.rdfmetadata.Specimen" column="specimenId" />

...