When I created nshvll.org, I wanted the ability to show the user a randomly ordered list of members. I also wanted to paginate the results, and I didn’t know if it was possible to have both. After a little research, I found that you can pass a seed to the mySQL RAND() function and have it return an identical list each time it’s called.

mySQL

Since I want each person to get a uniquely randomized list of members, I set a cookie with a seed which is randomly generated (1-100).

I set the seed in a before_filter in my ApplicationController.

Notice, I only set the cookie if it doesn’t exist. I also set the expiration of the cookie to 15 minutes because I don’t want the user to get the same list of members all the time.

Now, in the MembersController, I can pass that seed to mySQL and get a repeatable sequence of random records.

PostgreSQL

A little while after I built the site, I decided to move it from a personal VPS to Heroku. To do that, I needed to convert the database to PostgreSQL (or pay for a mySQL option). After a little research, I found out that PostgreSQL is a bit trickier than mySQL when it comes to using a seed with the random() function. You have to run a separate select query to set the seed before the query that gets the list of records.

PostgreSQL’s setseed() function requires a number between -1 and 1, so we need to switch to a random float instead of an integer in the ApplicationController

There we go. A repeatable, randomized set of records in mySQL or PostgreSQL.