Find a solution for method get_derives_from of EntitySnapshot

[iosonopersia]

Method get_derives_from of class EntitySnapshot has a big problem which must be addressed in future releases of oc_ocdm.

When references to other EntitySnaphots are found, either an EntitySnapshot instance already contained in the ProvSet is returned or a new one must be created and added to the ProvSet. In the latter case there's a problem: the constructor of a ProvEntity (superclass of EntitySnapshot) requires the prov_subject parameter, which the method is uncapable of retrieving as for now.

Should the prov_subject parameter be removed from the constructor's signature of ProvSet (by also adapting the rest of the code with the new assumption that a prov_subject could sometimes be unknown)? Or maybe there's a viable way to retrieve that information?

• If the parent snapshot(s) are stored in the same ProvSet instance, than there's no problem: add_se(None, uri) will not have to create any new entity and it will simply return the one that has that particular uri.
• If the parent snapshot(s) are NOT stored in the same ProvSet instance, we've currently no way to find out what their prov_subject is (calling add_se(None, uri) will create a new snapshot entity with None as prov_subject).

A TODO comment is put in place here:

oc_ocdm/oc_ocdm/prov/entities/entity_snapshot.py

Line 82 in 33e4c6e

# TODO: what is the prov_subject of these snapshots?