Question

I am designing a many-to-many relationship between two models: Cell and Isolator:

CREATE TABLE `isolator` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `name` varchar(127) NOT NULL,
  `surname` varchar(127) NOT NULL,
  PRIMARY KEY (`id`),
);

CREATE TABLE `cell` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `name` varchar(127) NOT NULL,
  `gen` varchar(127) NOT NULL,
  PRIMARY KEY (`id`),
);

CREATE TABLE `cell_isolators` (
  `cell_id` bigint(20) NOT NULL DEFAULT '0',
  `isolator_id` bigint(20) NOT NULL DEFAULT '0',
  PRIMARY KEY (`cell_id`,`isolator_id`),
  CONSTRAINT `cell_isolator_id` FOREIGN KEY (`isolator_id`) REFERENCES `isolator` (`id`),
  CONSTRAINT `isolator_cell_id` FOREIGN KEY (`cell_id`) REFERENCES `cell` (`id`) ON DELETE CASCADE
);

The client has asked me the possibility of specifying the order on which the list of isolators for a cell is shown, since it’s important for publication purposes.

What is the best approach to model that? I was thinking on adding a third field to the many-to-many relation (e.g. sort_order), but I would like to know if there are other alternatives.

Thanks!

Answer 1

The only way to do that in the general case is to add a column to “cell_isolators”.

The data type is application-dependent to a certain extent. I’ve seen this done with columns as integers, floats or decimals, and alphanumerics (varchar(n)).

Populating and maintaining it can be troublesome, though. That doesn’t have anything to do with the database design; it’s just painful to maintain the sort order for more than a few rows, especially if there are regular inserts to the table and changes to the sort order. Fortunately, that job usually falls to the user interface, not to the dbms.