def load(cls, rr, pk):
        '''
        Retrieves a document from the database, by primary key.
        '''
        if pk is None:
            return None
        d = rr.table(cls.table).get(pk).run()
        if d is None:
            return None
        doc = cls(rr, d)
        return doc
Retrieves a document from the database, by primary key.