'Returns all the entities of assertions for a subject, disregarding
whether the assertion still is valid or not.
:param subject_id: The identifier of the subject
:return: A possibly empty list of entity identifiers'

def entities(self, subject_id):

    try:
        return [i['entity_id'] for i in self._cache.find({'subject_id': subject_id})]
    except ValueError:
        return []
