modelformset question - interspersing extra forms among existing forms

Hi,

I have the following pseudo model that I will use as the basis of a modelformset:

class Entry:
    athlete = ForeignKey(athlete)
    category = ForeignKey(category)
    race_number = IntegerField

I have a page that displays each athlete's details so the choice of athlete has already been made. I want to add a formset that allows a user to specify a race_number for each of the possible categories available.

An obvious way would be for there to be one Entry for each athlete/category, but in practise this will result in a large entry table that is only ~10% actual data, with the rest being superfluous rows. I'd prefer to avoid this.

Let's assume that an athlete is already entered in 2 categories out of a possible 10. The queryset will return the 2 existing entries, and I can use extra=8 to add in the other possibilities. However, I am not yet done. I want to do the following and would like advice on how to accomplish this:

1. The category for each the 8 possible new entries needs to be assigned before displaying the form - I already know what they are and want to display these categories to the user, leaving the race_number as the only input field.
   
2. The categories have a particular (chronological) order that I want respected in displaying the forms to the user, so the 2 existing and 8 possible new categories need to be interspersed to follow the correct order.
3. When it comes time to save the formset, I want only those categories that have been changed by having a race_number assigned or removed to be saved. I want the entries where the race_number has been removed to remain in the entry table. I think I need a way to detect and remove the forms that have not been changed by the user before saving the formset.

Any pointers as to how to do this?

Thanks!
R.

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