def import_orm(self):
        #TODO: check docstring
        """ Import ORM classes for oedb access depending on input in config in
        self.config which is loaded from 'config_db_tables.cfg'
        
        Returns
        -------
        int
            Descr #TODO check type
        """

        orm = {}

        data_source = self.config['input_data_source']['input_data']
        mv_grid_districts_name = self.config[data_source]['mv_grid_districts']
        mv_stations_name = self.config[data_source]['mv_stations']
        lv_load_areas_name = self.config[data_source]['lv_load_areas']
        lv_grid_district_name = self.config[data_source]['lv_grid_district']
        lv_stations_name = self.config[data_source]['lv_stations']
        conv_generators_name = self.config[data_source]['conv_generators']
        re_generators_name = self.config[data_source]['re_generators']

        from egoio.db_tables import model_draft as orm_model_draft, \
            supply as orm_supply, \
            demand as orm_demand, \
            grid as orm_grid

        if data_source == 'model_draft':
            orm['orm_mv_grid_districts'] = orm_model_draft.__getattribute__(mv_grid_districts_name)
            orm['orm_mv_stations'] = orm_model_draft.__getattribute__(mv_stations_name)
            orm['orm_lv_load_areas'] = orm_model_draft.__getattribute__(lv_load_areas_name)
            orm['orm_lv_grid_district'] = orm_model_draft.__getattribute__(lv_grid_district_name)
            orm['orm_lv_stations'] = orm_model_draft.__getattribute__(lv_stations_name)
            orm['orm_conv_generators'] = orm_model_draft.__getattribute__(conv_generators_name)
            orm['orm_re_generators'] = orm_model_draft.__getattribute__(re_generators_name)
            orm['version_condition_mvgd'] = 1 == 1
            orm['version_condition_mv_stations'] = 1 == 1
            orm['version_condition_la'] = 1 == 1
            orm['version_condition_lvgd'] = 1 == 1
            orm['version_condition_mvlvst'] = 1 == 1
            orm['version_condition_re'] = 1 == 1
            orm['version_condition_conv'] = 1 == 1
        elif data_source == 'versioned':
            orm['orm_mv_grid_districts'] = orm_grid.__getattribute__(mv_grid_districts_name)
            orm['orm_mv_stations'] = orm_grid.__getattribute__(mv_stations_name)
            orm['orm_lv_load_areas'] = orm_demand.__getattribute__(lv_load_areas_name)
            orm['orm_lv_grid_district'] = orm_grid.__getattribute__(lv_grid_district_name)
            orm['orm_lv_stations'] = orm_grid.__getattribute__(lv_stations_name)
            orm['orm_conv_generators'] = orm_supply.__getattribute__(conv_generators_name)
            orm['orm_re_generators'] = orm_supply.__getattribute__(re_generators_name)
            orm['data_version'] = self.config[data_source]['version']
            orm['version_condition_mvgd'] =\
                orm['orm_mv_grid_districts'].version == orm['data_version']
            orm['version_condition_mv_stations'] = \
                orm['orm_mv_stations'].version == orm['data_version']
            orm['version_condition_la'] =\
                orm['orm_lv_load_areas'].version == orm['data_version']
            orm['version_condition_lvgd'] =\
                orm['orm_lv_grid_district'].version == orm['data_version']
            orm['version_condition_mvlvst'] =\
                orm['orm_lv_stations'].version == orm['data_version']
            orm['version_condition_re'] =\
                orm['orm_re_generators'].columns.version == orm['data_version']
            orm['version_condition_conv'] =\
                orm['orm_conv_generators'].columns.version == orm['data_version']
        else:
            logger.error("Invalid data source {} provided. Please re-check the file "
                         "`config_db_tables.cfg`".format(data_source))
            raise NameError("{} is no valid data source!".format(data_source))

        return orm