CollectionContainerPolicy containerPolicy = new CollectionContainerPolicy(ArrayList.class);
relationshipMapping.setContainerPolicy(containerPolicy);
relationshipMapping.setField(new XMLField("_relationships"));
relationshipMapping.setReferenceClass(Link.class);
XMLJavaTypeConverter converter = new XMLJavaTypeConverter(RelationshipLinkAdapter.class);
converter.initialize(relationshipMapping, event.getSession());
relationshipMapping.setConverter(converter);
descriptor.addMapping(relationshipMapping);
XMLCompositeObjectMapping hrefMapping = new XMLCompositeObjectMapping();
hrefMapping.setAttributeName("_persistence_href");