On entities that have only association keys as primary keys:

Calling BasicEntityPersister::exists() cause the invocation of ClassMetadata::getIdentifierValues() method.
This method returns associated entities as identifier values.
Then BasicEntityPersister::exists() tries to converts these objects to string, instead of searching their primary keys.

Some problems:

• getIdentifierValues have not access to external world (only its class metadata and the current entity).
• getIdentifierValues is uses in many places of doctrine

(sorry for my bad English)