Simple snippet how to save (update) related fields
class MovieAdmin(admin.ModelAdmin):
list_display = ('id', 'title')
save_on_top = True
def save_model(self, request, obj, form, change):
obj.imbd = get_imbd()
obj.save()
def save_related(self, request, form, formsets, change):
super(MovieAdmin, self).save_related(request, form, formsets, change)
obj = form.instance
if obj.comments.count() > 0:
obj.comments_count = obj.comments.count()
obj.save()