Agile Rails - How long does the Depot application hold onto the "Cart" array?

I'm walking through the wonderful "Agile Web Development for Rails" and in Chapter 8 on page 95 they build a Session model that has a physical table in the database and a "Cart" array that's linked to the unique Session id. So if the session key was an integer 123 the cart array must have a 'foreign key' value for the session_id = 123. The session is a 'real' database table in the MySQL database while the cart is an array that's linked by it's 'foreign key' named session_id. (This is the only way I can figure that it can hold the array?) My question is: This thing is stateless anyway...when (or does it ever) is the Cart array DUMPED from memory? Is it when the user logs off his browser? Is it after a certain time defined by the program? Is it after a certain time defined by the browser? What mechanism eventually dumps this array? Thank you, David

At the end of each request, all references to it disappear, and Ruby will eventually garbage collect it.

A new cart object is constructed for every incoming request on the session.