implement balance getting

uncloud_pay/selectors.py

@@ -0,0 +1,23 @@
+from django.utils import timezone
+from django.db import transaction
+
+from .models import *
+
+def get_payments_for_user(user):
+    payments = [ payment.amount for payment in Payment.objects.filter(owner=user) ]
+
+    return sum(payments)
+
+def get_spendings_for_user(user):
+    orders = Order.objects.filter(owner=user)
+
+    amount = 0
+    for order in orders:
+        amount += order.one_time_price
+        amount += order.recurring_price * order.count_used(when=timezone.now())
+
+    return amount
+
+@transaction.atomic
+def get_balance_for_user(user):
+    return get_payments_for_user(user) - get_spendings_for_user(user)