Hello Everybody

I bag you for help (it’s a third day of fight with this issue :(

The question is about one-to-zero-or-one association and lazy fetching (fetching on demand).

Let assume that we have 2 tables:



Additionally there is no strict 1 to 1 relation. Relation between Person and Note is: 1 to (0 or 1). So this is a reason why I cannot use ‘constrained’ HBM.XML mechanism.

I would like to have this:



But when I have a ‘note’ member in Person class (mapped in different ways) Hibernate always generate complex SQL selects involving more than one table (Person table):

I would expect (and this is my question!) to get here only simple SQL query against Person table and have additional SELECT in case of some operations on note class member.

I know that documentation stands:
Note that if constrained="false", proxying is impossible and Hibernate will eager fetch the association!

But on Hibernate forum I found many posts with hints that instrumentation could helpful for me. But, unfortunately, I don’t know how to use instrumentation in this particular case.

And maybe there is other way to handle such cases.

Note plase, that I would like to have this one-to-one association because it would be useful to use it with conjunction with runtime fetch strategies where at runtime I can decide in my Criteria queries, what I would like to load via JOINs.

Help pliiiiis.

Kind regards,
Adam

In person class try many-to-one mapping with not-null="false".

Regards,
Jitendra

Hello Jitendra

Unfortunately it doesn’t work either. Your suggestion works perfectly but for strict 1-1 relations only.

But I have a 1 to (0 or 1) relation. When there are corresponding records in Note table for ALL records from Person table everything works fine. But when I process a record from Person table with NO corresponding record in Note table, during accessing note’s property from Person object:



... I have ObjectNotFoundException exception.

I have tried all 4 combinations of “not-null” and “unique” HBM.XML settings (in many-to-one option) but every time I got a ObjectNotFoundException exceptions (for persons with no corresponding record in Note table).

Any other clues?

Regards,
Adam

PS.
Thank for your answer.

Here you go :-)

In many-to-one mapping you can put :

not-found="ignore"


This will not thru exception, but will return null.


Dun forget the credit :-)

Regards,
Jitendra

Hello Jitendra

Unfortunately it is not exactly what I am looking for :(

Once again: I have a one to zero-or-one relation between Person and Note entities. Additionally I want:
* to load Person.note member on demand (i.e. lazily)


When I applied your tip (not-found="ignore") like this:


... then when I try to load Person entity then Note entity is loaded eagerly. In other words with not-found="ignore" I have 2 SQL SELECTs (against Person and Note tables) or 1 complex joined SQL SELECT statement against those two tables (depending on fetch="join" or fetch="select" setting).

btw:
Thanks for your tips! Didn’t solve my problem but ... credited ;)

Kind regards,
Adam / Poland

Thanks for the credit. Though you did not say but i am assuming you are not getting exception so thats one part solved.

For making it fetch lazily, you need to explicitly set lazy="true" in many-to-one, as by default its set to lazy=false, hence eager fetching.

Hope it helps..

cheers!
Jitendra

Hello Jitendra

No, no, no :)
This your tip with lazy="true" doesn’t work either :(

btw:
There is no lazy="true" option for many-to-one mappings. Valid settings are: lazy="proxy|no-proxy|false". But I assume you meant “proxy”. And default setting for many-to-one is lazy=”proxy”, not lazy=”false” ;)

As far as I see it is not possible in Hibernate to gain what I am trying to get (and what was described by me, here in this forum thread).

Once again: I would like to have a following behavior:



For “line 1” I want to have SQL SELECT against PERSON table only.





In “line 2” I would like to have a second SQL SELECT shot to load data related to Note entity. Because I have 1 to (0 or 1) relation between Person and Note entities, it is possible that such SELECT will return no row and then person.getNote() will return null.

And I don’t want to be slapped by ObjectNotFoundException too :)

As I said, as far as I see it is not possible in Hibernate.

Want a bet? :) Polish Zubrowka is already waiting for someone who can show how to obtain such behavior in Hibernate :) ( http://en.wikipedia.org/wiki/Zubrowka )

Kind regards,
Adam

Which version of hibernate are you using. I am testing on hibernate 3.2.4.

I have the thing working.Will give you my test case later after verification :-) ..

Regards,
Jitendra

Hi Jitendra

All latest versions at my side: Hibernate 3.2.4.sp1.

btw:
You said that you have an appropriate piece of code which can handle my expectation. Could you copy+paste this code here? I would like to study your solution.

Kind regards,
Adam

Yeah will do that. Its on my home pc. I am in office right now. will do it later today.

cheers!
Jitendra

Here you go :
Test java code:


hbm.xml


Class is rather simple to write.

You will see lazy fetching using cglib. This is closest you can get to not getting the complete class.

*Do rate if this helps*

Regards,
Jitendra

http://www.hibernate.org/162.html

Looks like a discussion just for you :-).

cheers!
Jitendra

Hello Jitendra

Code presented by you works perfectly but ... you have changed database schema! :)

Schema presented by you is:


... but my schema is (like I have described in my initial post):



I cannot change my database schema (it’s a legacy db).
Many thanks for your suggestion but it is not solving my problem :(

And, of course, I know this page http://www.hibernate.org/162.html :) (but thanks for this address anyway)
Unfortunately I haven’t found a solution on that page too :(

Kind regards,
Adam

ah oh .. How do you expect these two to link if there is no shared key in your db ?

Join table ?? i haven't used that . but are you allowed to add table in your schema else there is no solution possible.

Regards,
Jitendra

Hi Jitendra

I don’t understand what do you mean.

I have a schema like this (Oracle database):



So I think that such situation can be described as “shared key”, am I right?

And such schema is easy to be mapped. You can find an example in official Hibernate Core documentation (have a look at http://www.hibernate.org/hib_docs/v3/reference/en/html_single/#assoc-unidirectional-121 ant a phrase “A unidirectional one-to-one association on a primary key”).
Solution described in the documentation obviously works but this solution is not lazy. And my original question in this forum thread was: “is it possible to gain a laziness for such two tables?”.

Kind regards,
Adam