OK, I think I may have found the answer. The legacy definition is:
CREATE TABLE `users_to_landlords` ( `user_id` smallint(6) NOT NULL DEFAULT '0', `landlord_id` smallint(6) NOT NULL DEFAULT '0', `prop_id` smallint(8) NOT NULL DEFAULT '0', PRIMARY KEY (`user_id`,`landlord_id`,`prop_id`) ) ENGINE=MyISAM DEFAULT CHARSET=latin1; So I think I can add "primarykey=['user_id', 'landlord_id', 'prop_id']" to the define_table() --