PR: https://github.com/hibernate/hibernate-orm/pull/1118
Just a quick comment to note that 'extended' enhancement is required in this use case. With it on, the resulting bytecode is:
0: aload_0 1: invokespecial #178 // Method $$_hibernate_read_id:()LChildKey; 4: invokevirtual #180 // Method ChildKey.$$_hibernate_read_parent:()Ljava/lang/String; 7: areturn