I added two additional segments to my test case where I set the `embedded` field to a stdClass instance and a GH611EmbeddedDocument instance, respectively. For stdClass, we predictably get an exception. A work-around would be to attempt preparation of objects through the PersistenceBuilder but catch exceptions and silently fall back the not preparing the value if the class isn't mapped. There doesn't seem to be a way to quickly check if a class is mapped or not, other than invoking `ClassMetadataFactory::doLoadMetadata()` and anticipating a MappingException. I'm not a fan of using try/catch for this purpose, though.