Question

I would like to know in Django how to get the average number of grouped elements in my table per element (without using .extra(…) if possible).

Concretely I have a table (DwarvesEatingCakes) consisting of Dwarves and Cakes. Each pair (dwarf, cake) is unique. I want to get the average number of cakes eaten by each dwarf.

The following code does not seem to work:

avg_cakeeaten_dict = DwarvesEatingCakes.values('dwarf').annotate(num_cake=Count('cake')).aggregate(avg_cake_eaten=Avg('num_cake'))

Thanks for the help!

Answer 1

The answer I ended up using is the following (based on Rob’s answer):

num_entries = DwarvesEatingCakes.objects.count() # get the number of entries
num_dwarves = DwarvesEatingCakes.objects.aggregate(num_dwarf=Count('dwarf', distinct=True))
return num_entries / float(num_dwarves['num_dwarf'])

Of course you also make your checks against divide by 0. There might be a standard way of doing this though since it is a relatively common place query.