rancher service & quota usage service

apps/herd/services/rancher_service.py

@@ -0,0 +1,13 @@
+from django.db.models import Sum, functions, Value
+
+
+class RancherService:
+
+    @staticmethod
+    def get_total_used_weight(rancher, sale_item):
+        return sale_item.objects.filter(
+            transaction__rancher=rancher,
+            transaction__transaction_status='success'
+        ).aggregate(
+            total_weight=functions.Coalesce(Sum('weight'), Value(0))
+        )['total_weight']