I'm surprised that this question apparently doesn't yet exist. If it does, please help me find it.
I want to use annotate (Count) and order_by, but I don't want to count every instance of a related object, only those that meet a certain criteron.
To wit, that I might list swallows by the number of green coconuts they have carried:
swallow.objects.annotate(num_coconuts=Count('coconuts_carried__husk__color = "green"').order_by('num_coconuts')