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If there's some info, which may be derived from the metamodel, the Handler should be able to persist the information as its private data using the MDR storage. Since the Handler is tightly coupled with a Storable, it seems logical to allow persistence within the Storable rather than creating another MDRepository object for that. Even in the later case, the Handler would need to persist the ID of the supplemental object.
Tomas, you did this task so please mark it as fixed if you are confident that it works.
The MDRBaseObject has method putProperty, removeProperty, getProperty to allow handlers to store persistently its own privite information. The stored object must be stroable using MDR serialization.