How can I restrict record creation in access based on a field value?

Question

I have a small access database I use to let students sign up for my afterschool classes (electronics, programming etc), I can only accept 30 people in each session.

At the moment they create their record and receive a message to learn how many people have already signed up (which I time-stamp in case of dropouts, first come first served).

However what I would like to do is prevent the record being created unless there is a slot available.

I've added a field on the form which says how many places are left, how can I stop access creating a new record when this value is <= 0?

Thanks in advance everyone

Answer 1

在窗体的OnCurrent事件中，插入以下代码行：

Me.AllowAdditions = (Me!PlacesLeft.Value > 0)