Hi All,

I am using Hibernate 3.2.1 with Java5 annotations and MySQL as database.
In my domain model I have an entity "ApprovalRequest" which has a unidirectional ManyToOne association to an entity "Image".



Per default hibernate seems to map this association as foreign key relation resulting in an "image_id" column in my "approvalrequest" table which is perfectly fine. Additionally a foreign key constraint is added to the "image_id" column.

My problem is that I have to delete Image entities which results in org.hibernate.exception.ConstraintViolationException because of the foreign key constraint on the "image_id" column.

Is it possible to instruct hibernate to reset the value of the "image_id" column to null if an "Image" entity is deleted or do I have to handle this in my code?

Hibernate may be doing many things. Some of them good, some of them crappy. I think you'll need to implement this on your own.


Now bash me. -- Harry

Hi Thomas,

first of all make clear, if Image really is an entity that is referenced by other entities. If not you should think about using a collection of Image's, see @CollectionOfElements.

If you think Image must be an entity you can add nullable = true to your @ManyToOne.

-Thomas

Hi ThoKes,

thank you for your help.

"Image" is definitly an entity in the system. It is one of the core entities.
And since "Image" is part of a subsystem and "ApprovalRequest" is part of another subsystem the association has to be unidirectional. Therefore there is no easy way in the code to remove the Image from the ApprovalRequest before deleting it.

I'll try the 'nullable=true' option. Thanks

Sorry ThoKes, but there seems to be no 'nullable' property available for the Annotation @ManyToOne. There is a property 'optional' which could be set to 'true' but has no effect to the generated DB schema.

Hi Thomas,

sorry for being unprecise, I meant nullable attribute of JoinColumn, pls see example below which should be like yours (Fahrzeug2 equal to your Image, referencing Halter2)


@Entity
public class Halter2 {
@Id
@GeneratedValue (strategy = GenerationType.AUTO)
private long id;

@OneToMany (targetEntity = Fahrzeug2.class, mappedBy = "halter", cascade = CascadeType.ALL)
private Set<Fahrzeug2> fahrzeuge;

...
}


@Entity
public class Fahrzeug2 {
@Id
@GeneratedValue (strategy = GenerationType.AUTO)
private long id;

@ManyToOne ( targetEntity = Halter2.class, fetch = FetchType.LAZY)
@JoinColumn (name = "halter_id", nullable= true)
private Halter2 halter;

@Column (name = "kennzeichen")
private String kennzeichen;
...
}

create table Fahrzeug2 (id number(19,0) not null, kennzeichen varchar2(255), halter_id number(19,0), primary key (id))
19:49:43,953 DEBUG org.hibernate.tool.hbm2ddl.SchemaExport: create table Halter2 (id number(19,0) not null, primary key (id))
19:49:44,015 DEBUG org.hibernate.tool.hbm2ddl.SchemaExport: alter table Fahrzeug2 add constraint FK2DFFC4D0F16DC5AE foreign key (halter_id) references Halter2
19:49:44,031 DEBUG org.hibernate.tool.hbm2ddl.SchemaExport: create sequence hibernate_sequence
19:49:44,031 INFO org.hibernate.tool.hbm2ddl.SchemaExport: schema export complete

As you can see, the FK contraint now has no NOT NULL clause and I could delete a row from Fahrzeug2.


-Thomas

Hi Thomas,

you're right. Your sample is working perfectly. But my problem is that my association is unidirectional. As soon as you remove the @OneToMany end from the Halter2 class you should run into the same problem as I do.
I've written a little test for your sample and have been able to reproduce my problem. The test succeeds if the association is bidirectional but fails if it is unidirectional.

Hi again,

I've digged a bit deeper into my problem. It seems that the JoinColumn annotation is not necessary in my case. The problem seems to be that if the association is unidirectional it is not possible to declare a cascade behaviour for the not navigable end of the association. I.e. the test succeeds if the Halter2 class simply declares



No getter or setters are necessary for the member 'fahrzeuge'. Just the 'cascade' annotation property has to be set.

- Thomas

Hmmmh,

maybe I still don't understand your problem. If ApprovalRequest hast FK to Image then you cannot delete an reference Image because of the FK. You could update referencing ApprovalRequest to null and then delete the referenced Image. Some database have CASCADE NULLIFY feature.

For the @OneToMany there is optional=true attribute, but your problem in your first post was @ManyToOne.

BTW, if you commit @JoinColumn Hibernate concatenates name of join column from ref. tablen and "_id".

-Thomas

ommit (not commit) :-)