Active users in the last 5 minutes

gistfile1.rb

# Inside the controller code that fetches the logged in user for every request
# Note: Trying to call EXPIRE against a key that already has an associated timeout will not change the timeout of the key, but will just return 0

redis = Redis.new
minutes = 5
seconds = minutes * 60

redis.multi do
  redis.sadd("active-users", user.id)
  redis.expire("active-users", seconds)
end

# To find current active users
User.find(redis.smembers("active-users"))

This only works in Redis 2.1.3 or greater. If you're running an earlier version you will have to delete the 'active-users' key on a cron job or scheduled job.