Having problem with validates_presence_of fields in form when trying to also use ActiveScaffold (x-posted at ActiveScaffold group)

I have an events listing site that includes the models Event and
DateAndTime, so an event that happens more than once (like a high
school play) can have more than one date_and_time. The Event model
has_many date_and_times, and the DateAndTime model belongs_to event.
The Event model validates_presence_of two submission form variables,
the_date and the_time, and those variable values then get used to
create a new date_and_time object once the form is submitted.

My ActiveScaffold problem is that when the update form for the Event
model displays, it shows all the correct dates and times in drop-down
lists (with the ability to either edit or delete them) PLUS an *extra*
date and time field that's set by default to the current date and
time, with no way to delete that field. The update form is created
with this new additional date field required, even though the event
object already has other date_and_time objects associated with it.

Any ideas how I can make my Event update form display without the new
date and time non-deletable field in ActiveScaffold, while still
requiring date and time submissions on the public site's Event
creation form?

Thank you.