First, I have readed the reference and the faqs, but i cant find a solution of my problem.
There is an existing ddbb schema and existing classes. I cant touch them.

There is a class A (parent) with a single reference of a class B (child). The class B may have no parent of type class A (there is another type of parent).

But in the ddbb the table A dont have a key to the table B. Is the B one that have the FKA.

I cant use the one-to-one because the PKA & PKB arent equivalent .
I cant use the many-to-one, because the FK is in the wrong class.

Is there a solution without modify the clases or ddbb schema?

Thanks for read my poor english:

Miki