Hi,

I'm having a problem lazy loading a one-to-one association. I'm on NHibernate 1.2. Using the mapping file below, NHibernate is adding the one-to-one class loading into the SQL select statement for the parent class, but is then attempting to fetch it again for each object separately. I don't want it to do either.

mapping files: (snipped for clarity)


I've checked there is no code trying to access the one-to-one property. I read a post saying that one-to-ones couldn't be lazy loaded, but then several others stating they could. Can anyone help with the correct setup to get lazy loading?

Cheers
Rory

As per the document here: http://www.hibernate.org/162.html, I changed the one-to-one mapping to:

This causes no loading to occur, however when I then go to access the property I get the exception "NHibernate.ObjectNotFoundException: No row with the given identifier exists: 141013" because the association is on a foreign key. Adding the property-ref attribute back causes a select fetch on the initial load.

So the question now is: how to get lazy load with a one-to-one association on a foreign key?

From the article above:

I'm confused here. I thought in NHibernate at least, the proxy for B does know when the setter for Cee is called, because the property must be virtual in your entity class and the proxy overrides it. So, why can't the proxy's override for the Cee property setter attempt to load the value from the database on first access? That way, time won't be wasted on rarely-touched, non-mandatory one-to-one properties.

I seem to remember the answer to a similar question about "why cant a proxy getter/setter initialize something as needed" was "the proxies are generic from Castle, they don't know anything about NHibernate in particular". If that's the case, that's really too bad ...

The bottom line is that (N)Hibernate does not support lazy loading of one-to-one relations. There are several posts and explanations on why. There is one post out there by yours truly that says that it's possible and how, but you should ignore it as is was purely the result of a brain-fart in someone who was a novice a the time.

Essentially, if you want to have lazy loading don't use one-to-one, use one-to-many.

Symon.

Thanks for the replies, I came to the same conclusion myself. It is possible if the objects are constrained (there will always be two) but in my case this wasn't always true.

I still had problems with the loading of the object being done in a seperate query, rather than the original object query (this wouldn't be so bad but I could be querying for anything up to a few thousand records). It seems that using property-ref="" on the one-to-one causes it to use another select (despite also adding an outer-join to the original query). In the end I changed the mapping file for the child object to use the foreign key as the identifier, coupling that with an outer-join="true" on the one-to-one mapping got the result I wanted (everything happens in one select).

I have the same problem but i can;t remove the property-ref.
Consider this scenatio :
I have one-to-one relation between Product and ProductBasic BUT at database level i have one-to-many between
PRODUCT and PRODUCTBASIC tables as history is mainted in the same table :

PRODUCT ----> PID (PK)
PRODUCTBASIC ---> PID (FK), USECODE , STARTTIMESTAMP forms Composite-key

PRODUCT
PID PNAME
1 Pen

PRODUCTBASIC
PID USECODE STARTTIME ENDTIME
1 100 06-MAY-2009 11:00 07-MAY-2009 10:00
1 100 07-MAY-2009 10:00 31-DEC-9999 00:00

Now while querying for ProductBasic, i will always fetch the row which satisfies the condition STARTTIME<CURRENTTIME<ENDTIME
and this will surely return me only one row.

Ideally the generated sql query should be :

select * from PRODUCT product left outer join PRODUCTBASIC productbasic on '100'=productbasic.USETYPE
and product.PID=productbasic.PID
and (
productbasic.STARTTIME<'09-MAY-2009 10:00'
and '09-MAY-2009 10:00'<productbasic.ENDTIME
)
where
product.PID='1'

PLEASE NOTE, I am not satisfying the relationship by using composite key only, hence i can;t remove property-ref...

I have this requirement because i use filter on ProductBasic. and now when this ProductBasic is fetched always (non-lazyly) it doesn't
apply my filter.

I managed the problem by using @ManyToOne, and avoided the EntityNotFoundException using the workaround described in http://opensource.atlassian.com/project ... e/HHH-2753