If a data model includes one-to-one relationships that are optional/nullable, what's the best way to represent those so that the second-level cache will do the right thing?

For example, a Car may have zero or one Stereo, but a Stereo will always belong to a Car:



I've tried a lot of variations on this, and I've found that the second level cache (EHCache) handles this association right when the owner (Car) and the owned (Stereo) both exist. If the Car has no Stereo, however, I see a query for the missing Stereo every time I pull it out of the cache. I.e. it doesn't seem like the cache can store a null to say "this Car has no Stereo, so don't bother asking about it ..."

The caching works right for one-to-one relationships where optional=false.

The only way I've found to get the right behavior for this case is to model the relationship as a one-to-many where my business logic just maintains it as a one-to-one. When I do this, I can set up the cache for the entities and the collection, and I don't see extra queries for the missing child. This works ok, but feels like a hack. Is there a cleaner way to model this?[/code]

The problem here is that the relation is defined on primary keys so even though a Car instance is in cache HB has no way to understand if the car has a stereo. You will not have this problem if the relation is mapped with a foreign key. In addition, you will have to dictate select fetching or, as per my understanding, hibernate ignores your cache settings. On top of that, you should advice hibernate to cache the relationship as well as the entities. All I said can be put in the following example:




and



I hope this helps.

Farzad-

Thanks for the help, this is very useful.

So this approach requires that the "owner" table (Car) be modified so that it has a foreign key column that points to the child (CarStereo). Hibernate knows that a Car has no CarStereo because its FK column is null.

Is there any way to get the same behavior without adding a FK column? If Cars were always retrieved using an outer join against CarStereo, Hibernate would know immediately when a Car has no CarStereo. It would be nice if it could store this boolean information on the Car in a way that could be cached. I.e. if the serialized representation in EHCache could have a flag that says "this association is null".

Thanks again

I would like to see this too as caching comes as a mystery to me each time but as far as I know hibernate does not serialize objects into cache but copies the values and where possible only ids. So, in the case of this example it copies a hypothetical car object as [1, "A Car", 1], the last 1 being the id of a Stereo object in my example. Having used the primary key hibernate does not store the last 1 (only my guess, I actually like to verify this). However, your point is still valid here and to the best of my knowledge selects that don't return a result are a little bit costly and can cause lock escalations. So there is definitely an interest to solve this. I will probably examine the issue in more details and open an issue if there is a room for improvement.


Farzad-

Hi,
I know this is not very related to this post, but I am having a problem using OneToOne relation with a code very similar to the one posted here.
I switched to Hibernate 3.6 and ever since I started getting this exception:

Caused by: org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [com.todacell.ui.model.bo.campaign.TargetDays#756]
at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:433)
at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:233)
at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:285)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152)
at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1090)
at org.hibernate.impl.SessionImpl.internalLoad(SessionImpl.java:1038)
at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:630)
at org.hibernate.type.EntityType.resolve(EntityType.java:438)
at org.hibernate.engine.TwoPhaseLoad.initializeEntity(TwoPhaseLoad.java:139)
at org.hibernate.loader.Loader.initializeEntitiesAndCollections(Loader.java:982)
at org.hibernate.loader.Loader.doQuery(Loader.java:857)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:274)
at org.hibernate.loader.Loader.loadEntity(Loader.java:2037)
at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:86)
at org.hibernate.loader.entity.AbstractEntityLoader.load(AbstractEntityLoader.java:76)
at org.hibernate.persister.entity.AbstractEntityPersister.load(AbstractEntityPersister.java:3268)
at org.hibernate.event.def.DefaultLoadEventListener.loadFromDatasource(DefaultLoadEventListener.java:496)
at org.hibernate.event.def.DefaultLoadEventListener.doLoad(DefaultLoadEventListener.java:477)
at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:227)
at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:285)
at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152)
at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1090)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:1005)
at org.hibernate.impl.SessionImpl.get(SessionImpl.java:998)
at com.todacell.ui.model.dao.BaseDAO.getObject(BaseDAO.java:62)
... 69 more

Does anyone has a clue why Hibernate 3.6 gives this exception even though I am using "optional=true"?
Is there a workaround for this problem?

@bashan: Were you able to solve this problem. I have many-one mapping and this worked fine until I switch to Hibernate-3.6.
Following is the exception while accessing the object:


But in the database there is a record for corresponding ID#12345678. Also, when I change mapping record to a different Id, I am able to access the object without any exceptions.