Django sum a field based on foreign key

from django.db.models import Count
Games.objects.annotate(num_players=Count('players')).filter(num_players__gt=10)

 MyModel.objects.values('user', 'category' 
     ).annotate(max_rank=Max('status__rank'))

class CharityFilter(SimpleListFilter):
    title = _('Charity')
    parameter_name = 'charity'
    def lookups(self, request, model_admin):
        queryset = model_admin.queryset(request).filter(organisation__type='CHARITY')
        return queryset.values_list('organisation__pk','organisation__name').order_by('organisation__name')

    def queryset(self, request, queryset):
        if self.value():
          return queryset.filter(organisation=self.value())

class DomainDefault(object):
    def set_context(self, serializer_field):
        view = serializer_field.context['view']
        request = serializer_field.context['request']
        self.domain = ...#determine the domain based on request+view

    def __call__(self):
        return self.domain


class RRsetSerializer(serializers.ModelSerializer):
    domain = serializers.HiddenField(default=serializers.CreateOnlyDefault(DomainDefault()))