How to create a unique identifier for tracking?

How do you create a unique identifier for users for tracking purposes?

• I would like to be able to add a user referal params on all invite links sent out from my site... example: http://site.com/invited_by?=ajraux How can I generate a code that is short like "ajraux" for all users on my site? Does it need to be a field in the database? Can it be created on the spot? How are web companies doing this? Thanks

• Answer:

  You could create random numbers and encode them in base-36, something simple like this: rand(1e12).to_s(36) Generate one for each user on first use and store it with the user. Add a unique constraint on your random token (in both your model and the database) and generate a new one if you get a uniqueness violation. You might want to log a warning somewhere that you'll see it if you need to try more than, say, five times to get a unique value; if you start getting a lot of warnings then bump that 1e12 up to 1e15 (or higher). That would give you a random looking token attached to each user, the tokens would be URL-safe, they're quick and easy to generate, you shouldn't get that many collisions, and it will be easy to backtrack from a token to the user.