Triggering a set field

[brainonastick]

I have a web form which registers students from courses stored in a Course Information DB. The Course Information DB displays course information on the website. The registration records are stored in a Registrations DB. I need the Course Capacity in the Course Information DB to be updated whenever someone registers. To do this I need to trigger the capacity in the Registrations DB to be pasted into the Capacity in the Course Information DB. The reason I need to do this is because if I set up a calculation (pipe) field between the 2 DBs the website wont read it for some reason. I have to use a native field from Course Information to display the capacity on the website.

I cant seem to find the right trigger - the Web Registration Db gets updated at the server when a web form is submitted. Do any of the triggers react to this kind of activity?

[Fitch]

Script triggers don't work on the web.* Is this IWP?

*"in Instant Web Publishing, only scripts can activate script triggers. For example, if an Instant Web Publishing user clicks a field that has an OnObjectEnter script trigger, the trigger is not activated. However, if a script causes the focus to move to the field, then the OnObjectEnter script trigger is activated." -- IWP docs