Until Hibernate 4.2 it was possible to use embeddable classes as MapKey in OneToMany Relations.
Now there are problems while parsing the metamodel (=when building the session-factory).
Follwing exception raises in testcase that will follow asap as pull-request:

org.hibernate.testing.junit4.BaseCoreFunctionalTestCase#buildSessionFactory
org.hibernate.testing.junit4.CallbackException: org.hibernate.testing.junit4.BaseCoreFunctionalTestCase#buildSessionFactory
at org.hibernate.testing.junit4.TestClassMetadata.performCallbackInvocation(TestClassMetadata.java:212)
at org.hibernate.testing.junit4.TestClassMetadata.invokeCallback(TestClassMetadata.java:196)
at org.hibernate.testing.junit4.TestClassMetadata.performCallbacks(TestClassMetadata.java:190)
at org.hibernate.testing.junit4.TestClassMetadata.performBeforeClassCallbacks(TestClassMetadata.java:181)
at org.hibernate.testing.junit4.BeforeClassCallbackHandler.evaluate(BeforeClassCallbackHandler.java:42)
at org.hibernate.testing.junit4.AfterClassCallbackHandler.evaluate(AfterClassCallbackHandler.java:42)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: java.lang.NullPointerException
at org.hibernate.loader.plan.build.internal.AbstractLoadPlanBuildingAssociationVisitationStrategy.foundCircularAssociation(AbstractLoadPlanBuildingAssociationVisitationStrategy.java:689)
at org.hibernate.persister.walking.spi.MetamodelGraphWalker.visitAttributeDefinition(MetamodelGraphWalker.java:163)
at org.hibernate.persister.walking.spi.MetamodelGraphWalker.visitAttributes(MetamodelGraphWalker.java:148)
at org.hibernate.persister.walking.spi.MetamodelGraphWalker.visitCompositeDefinition(MetamodelGraphWalker.java:216)
at org.hibernate.persister.walking.spi.MetamodelGraphWalker.visitCollectionIndex(MetamodelGraphWalker.java:247)
at org.hibernate.persister.walking.spi.MetamodelGraphWalker.visitCollectionDefinition(MetamodelGraphWalker.java:224)
at org.hibernate.persister.walking.spi.MetamodelGraphWalker.visitCollection(MetamodelGraphWalker.java:89)
at org.hibernate.loader.plan.build.spi.MetamodelDrivenLoadPlanBuilder.buildRootCollectionLoadPlan(MetamodelDrivenLoadPlanBuilder.java:71)
at org.hibernate.loader.collection.plan.AbstractLoadPlanBasedCollectionInitializer.<init>(AbstractLoadPlanBasedCollectionInitializer.java:76)
at org.hibernate.loader.collection.plan.CollectionLoader.<init>(CollectionLoader.java:112)
at org.hibernate.loader.collection.plan.CollectionLoader$Builder.byKey(CollectionLoader.java:105)
at org.hibernate.loader.collection.plan.AbstractBatchingCollectionInitializerBuilder.buildNonBatchingLoader(AbstractBatchingCollectionInitializerBuilder.java:45)
at org.hibernate.loader.collection.BatchingCollectionInitializerBuilder.createBatchingOneToManyInitializer(BatchingCollectionInitializerBuilder.java:102)
at org.hibernate.persister.collection.OneToManyPersister.createCollectionInitializer(OneToManyPersister.java:479)
at org.hibernate.persister.collection.AbstractCollectionPersister.postInstantiate(AbstractCollectionPersister.java:677)
at org.hibernate.internal.SessionFactoryImpl.<init>(SessionFactoryImpl.java:484)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1857)
at org.hibernate.testing.junit4.BaseCoreFunctionalTestCase.buildSessionFactory(BaseCoreFunctionalTestCase.java:148)
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.hibernate.testing.junit4.TestClassMetadata.performCallbackInvocation(TestClassMetadata.java:209)
... 12 more