This is called immediately before the items are written to the
        database. pkgen is passed in to allow last-minute resolving of ids.

def prepare(cls, session, pkgen, items):
        """This is called immediately before the items are written to the
        database. pkgen is passed in to allow last-minute resolving of ids.
        """
        for item in cls.merge(session, items):
            if hasattr(item, "id"):
                item.id.resolve(id=pkgen.get(cls), is_new=True)
            yield cls.to_dict(item)