when i put it on many-to-one, i was getting these errors.

java.lang.NullPointerException
at org.hibernate.util.StringHelper.qualify(StringHelper.java:264)
at org.hibernate.util.StringHelper.qualify(StringHelper.java:278)
at org.hibernate.loader.JoinWalker.walkCompositeElementTree(JoinWalker.java:499)
at org.hibernate.loader.JoinWalker.walkCollectionTree(JoinWalker.java:310)
at org.hibernate.loader.JoinWalker.walkCollectionTree(JoinWalker.java:255)
at org.hibernate.loader.collection.BasicCollectionJoinWalker.<init>(BasicCollectionJoinWalker.java:47)
at org.hibernate.loader.collection.BasicCollectionLoader.<init>(BasicCollectionLoader.java:53)
at org.hibernate.loader.collection.BasicCollectionLoader.<init>(BasicCollectionLoader.java:40)
at org.hibernate.loader.collection.BatchingCollectionInitializer.createBatchingCollectionInitializer(BatchingCollectionInitializer.java:87)
at org.hibernate.persister.collection.BasicCollectionPersister.createCollectionInitializer(BasicCollectionPersister.java:297)
at org.hibernate.persister.collection.AbstractCollectionPersister.postInstantiate(AbstractCollectionPersister.java:541)
at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:294)