When defining relations, you specify the join column(s) that carries the FK information. But let's say we use lazy fetching and are interested in the FK value without resolving the relation. I did not find how to express this with JPA annotations. Any ideas? Thanks in advance!

If it's not clear yet, I try to express it as an example using /*non working*/ pseudo annotations:
Variation 1:

@Column(name="customer_id")
int customerId;
@OneToOne
// non-existent annotation:
@JoinProperty(name="customerId")
Customer customer;


Variation 2:

@Column(name="customer_id")
int customerId;

@OneToOne
// Mapping exception: repeated column in mapping:
@JoinColumn(name="customer_id")
Customer customer;

You could probably use 2.
But be sure to set insertable=false, updateable=false .. since otherwise it will complain about duplicate columns.

Because the relation must be updateable, I concluded this is not an option.

Ok, you leave this on one of the properties - but code in such way that when association changes, it also changes int value. The other way around is probably more difficult to write - but can be done with some EventListener impl.

Thank you for your replies.
I am not sure if I understood you correctly. Does your description include resolving the relation? I need it without resilving the relation. Thanks.

map your property only once

in Hibernate (this is (might not be) not true for bytecode enhanced based persistence engine),

child.getParent().getId() (which is the FK value you're looking for) does not load the parent association

emmanuel, thank you for your reply. Did you mean Hibernate Core when you wrote of Hibernate? A solution working with JPA would be prefered though.

This will work on Hibernate Core and Hibernate EntityManager.
I cannot guaranty for others JPA implementations since most JDO based engine use buildtime enhancement rather than proxy for lazy loading because of the way the JDO spec has been designed.