EM#find() with association as PK

Say I have a association as primary key in my entity, and I want to find it by id.
I'm calling EntityManager#find("MyEntity1", MyEntity2 $entity),
then it calls UnitOfWork#tryGetById() wich makes implode of sorted ids, and that is error, because sortedId contains object.

I can implement __toString in my entity, but I think this is work of ORM.
What are your thoughts?
I could fix that, but is there some instruction on Doctrine's workflow?