Mathematical Operations on Django Annotations.

Question

I have a Django model that defines a TimeSlot. Each TimeSlot can hold a certain number of users (TimeSlot.spots). Each TimeSlot also has a certain number of users already held in it (a many to many field, TimeSlot.participants.

When I pass to the template that displays the available TimeSlots to the user, I annotate with TimeSlot.objects.annotate(Count('participants')),which gives the number of users currently held by the TimeSlot as participants__count.

However, what I really want is the number of spots remaining, the capacity (TimeSlot.spots) minus the number currently held (participants__count). How can I annotate another field with this new number, so I can pass it to the template?

Answer 1

Not possible with only an annotation. I'd create a method on the model which does the annotation, and then subtract that from your TimeSlot.spots value. This will use more database queries, but thats your only option. Or I guess you could drop down to raw SQL...