Using @NaturalIdCache on an entity class having a ManyToOne relation as natural id, causes following exception when reattaching an unmodified instance:
org.hibernate.PropertyAccessException: could not get a field value by reflection getter of org.hibernate.test.naturalid.mutable.cached.A.oid
at org.hibernate.property.DirectPropertyAccessor$DirectGetter.get(DirectPropertyAccessor.java:62)
at org.hibernate.tuple.entity.AbstractEntityTuplizer.getIdentifier(AbstractEntityTuplizer.java:341)
at org.hibernate.persister.entity.AbstractEntityPersister.getIdentifier(AbstractEntityPersister.java:4425)
at org.hibernate.persister.entity.AbstractEntityPersister.isTransient(AbstractEntityPersister.java:4147)
at org.hibernate.engine.internal.ForeignKeys.isTransient(ForeignKeys.java:209)
at org.hibernate.engine.internal.ForeignKeys.getEntityIdentifierIfNotUnsaved(ForeignKeys.java:248)
at org.hibernate.type.ManyToOneType.disassemble(ManyToOneType.java:214)
at org.hibernate.cache.spi.NaturalIdCacheKey.<init>(NaturalIdCacheKey.java:84)
at org.hibernate.engine.internal.StatefulPersistenceContext$1.removeSharedNaturalIdCrossReference(StatefulPersistenceContext.java:1991)
at org.hibernate.persister.entity.AbstractEntityPersister.handleNaturalIdReattachment(AbstractEntityPersister.java:4134)
at org.hibernate.persister.entity.AbstractEntityPersister.afterReassociate(AbstractEntityPersister.java:4106)
at org.hibernate.event.internal.AbstractReassociateEventListener.reassociate(AbstractReassociateEventListener.java:100)
at org.hibernate.event.internal.DefaultLockEventListener.onLock(DefaultLockEventListener.java:81)
at org.hibernate.internal.SessionImpl.fireLock(SessionImpl.java:811)
at org.hibernate.internal.SessionImpl.fireLock(SessionImpl.java:804)
at org.hibernate.internal.SessionImpl.access$11(SessionImpl.java:803)
at org.hibernate.internal.SessionImpl$LockRequestImpl.lock(SessionImpl.java:2365)
at org.hibernate.test.naturalid.mutable.cached.CachedMutableNaturalIdTest.testReattachementUnmodifiedInstance(CachedMutableNaturalIdTest.java:309)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
at org.hibernate.testing.junit4.ExtendedFrameworkMethod.invokeExplosively(ExtendedFrameworkMethod.java:63)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:30)
at org.junit.internal.runners.statements.FailOnTimeout$StatementThread.run(FailOnTimeout.java:62)
Caused by: java.lang.IllegalArgumentException: Can not set long field org.hibernate.test.naturalid.mutable.cached.A.oid to java.lang.Long
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(Unknown Source)
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(Unknown Source)
at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(Unknown Source)
at sun.reflect.UnsafeLongFieldAccessorImpl.getLong(Unknown Source)
at sun.reflect.UnsafeLongFieldAccessorImpl.get(Unknown Source)
at java.lang.reflect.Field.get(Unknown Source)
at org.hibernate.property.DirectPropertyAccessor$DirectGetter.get(DirectPropertyAccessor.java:59)
... 29 more

Pull-request with testcase will follow asap.