Determine which user is online and active in the last 30 minutes

This is what I did to determine how many users and which user is online and active in the last 30 minutes from Rails. I went through http://groups.google.com/group/rubyonrails-talk to find out how other people dealt with this issue. Most people deal with this through session with active_record_store. I think this way is quite reasonable because each time the user request to our pages, it will create a new record or update the existing record. This would allow me and other people interferes to determine which user is online.

To configure to use active_record_store with session, go to see Agile Web Development with Rails 2nd edition on page 99.

Some people are likely to interfere by adding the user_id and host column in the table sessions. They use before_filter on the application controller. If they found the session_id in the table is the same as session.session_id, they update the column user_id with session[:user_id]. To access to the sessions table, it must use this syntax CGI::Session::ActiveRecordStore::Session. I try with this syntax but could not update my fields anyway. It is difficult to interfere with code the rails creator use to write a record to the sessions table. It is not the normal active record model that I expected. So, I come up with another solution by creating a new model called Session that maps it to the sessions table. This way I could update the record in the table. But I still face with another problem. I need to write to the table whenever the user first request to my page with the new column "host" that record ip address. I don't know how to interfere with this.

Therefore, I find another solution that seems to be not so good, but reasonable. I create a new table that map to the sessions table called active_users that has the following fields: id, session_id, user_id, host, url, created_at, updated_at. I use after_filter on the application controller to update or create a new record in this table by looking into the session_id in the table first. If it is already exists, I just update the host, user_id, and url. Otherwise, I create a new row in this table. I also create a trigger on the sessions table. When the user deletes the records, it also deletes record in active_users also. I think I should need a cron job that removes all unnecessary session records every 1 hour. Oh, there is still another problem, when the user log out, don't forget to delete records in active_users table. There is no need for the sessions table because it will be done automatically. And, that's it. I create two methods in the User model.

def self.total_online
sql = "Select Count(Distinct user_id) as total " +
"From active_users " +
"Where updated_at > ?"
ActiveUser.find_by_sql([sql, 30.minutes.ago])[0].total.to_i
end

def self.online?(user_id)
sql = "Select * from active_users " +
"Where user_id = ? " +
"And updated_at > ?"
ActiveUser.find_by_sql([sql, user_id, 30.minutes.ago]).length >= 1
end