def _execute(self, uow):
        self._session = uow.session
        sql = '''select fields.id, fields.name
        from fields left join items_fields on fields.id = items_fields.field_id
        where items_fields.item_id is NULL
        '''
        hangingFields = self._session.query(db.Field).from_statement(sql).all()
        count = len(hangingFields)
        for field in hangingFields:
            self._session.delete(field)
        if count > 0:
            self._session.commit()
        return count