From adb57c55ca0c439a0577ebb1a0c24fbb678350ff Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timoth=C3=A9e=20Floure?= <timothee.floure@posteo.net>
Date: Fri, 28 Feb 2020 09:58:01 +0100
Subject: [PATCH] Revamp generate-bills logic to avoid overlapping

---
 .../management/commands/generate-bills.py     | 69 ++++++++++++-------
 1 file changed, 44 insertions(+), 25 deletions(-)

diff --git a/uncloud/uncloud_pay/management/commands/generate-bills.py b/uncloud/uncloud_pay/management/commands/generate-bills.py
index 92075ce..aad7a82 100644
--- a/uncloud/uncloud_pay/management/commands/generate-bills.py
+++ b/uncloud/uncloud_pay/management/commands/generate-bills.py
@@ -1,48 +1,67 @@
 from django.core.management.base import BaseCommand
 from uncloud_auth.models import User
 from uncloud_pay.models import Order, Bill
+from django.core.exceptions import ObjectDoesNotExist
 
 from datetime import timedelta
 from django.utils import timezone
 
+BILL_PAYMENT_DELAY=timedelta(days=10)
+
 class Command(BaseCommand):
     help = 'Generate bills and charge customers if necessary.'
 
     def add_arguments(self, parser):
         pass
 
-    # TODO: check for existing bills
     def handle(self, *args, **options):
-        customers = User.objects.all()
-        print("Processing {} users.".format(customers.count()))
-        for customer in customers:
-            orders = Order.objects.filter(owner=customer)
+        users = User.objects.all()
+        print("Processing {} users.".format(users.count()))
 
-            # Pay all non-billed usage untill now.
-            bill_starting_date = timezone.now()
-            bill_ending_date = timezone.now()
+        for user in users:
+            # Fetch all the orders of a customer.
+            orders = Order.objects.filter(owner=user)
 
-            billed_orders = []
+            # Default values for next bill (if any). Only saved at the end of
+            # this method, if relevant.
+            next_bill = Bill(owner=user,
+                       starting_date=timezone.now(), # Will be set to oldest unpaid order (means unpaid starting date).
+                       ending_date=timezone.now(), # Bill covers everything until today.
+                       due_date=timezone.now() + BILL_PAYMENT_DELAY)
+
+            unpaid_orders = [] # Store orders in need of a payment.
             for order in orders:
-                print(order)
-                if True: # FIXME
-                    billed_orders.append(order)
+                # Only bill if there is an 'unpaid period' on an active order.
+                # XXX: Assume everything before latest bill is paid. => might be dangerous.
+                try:
+                    previous_bill = order.bill.latest('ending_date')
+                except ObjectDoesNotExist:
+                    previous_bill = None
 
-                    # Update starting date if need be.
-                    if order.starting_date < bill_starting_date:
-                        bill_starting_date = order.starting_date
+                is_unpaid_period = True
+                if order.ending_date and previous_bill != None:
+                    is_unpaid_period = previous_bill.ending_date < order.ending_date
 
-            if len(billed_orders) > 0:
-              bill = Bill(owner=customer,
-                       starting_date=bill_starting_date,
-                       ending_date=bill_starting_date,
-                       due_date=timezone.now() + timedelta(days=10))
-              bill.save()
+                if is_unpaid_period:
+                    # Update bill starting date to match period.
+                    if previous_bill == None:
+                        next_bill.starting_date = order.starting_date
+                    elif previous_bill.ending_date < next_bill.starting_date:
+                        next_bill.starting_date = previous_bill.ending_date
 
-              for order in billed_orders:
-                  print(order)
-                  order.bill.add(bill)
+                    # Add order to bill
+                    unpaid_orders.append(order)
 
-              print("Created bill {} for user {}".format(bill.uuid, customer.username))
+            # Save next_bill if it contains any unpaid product.
+            if len(unpaid_orders) > 0:
+              next_bill.save()
 
+              # It is not possible to register many-to-many relationship before
+              # the two end-objects are saved in database.
+              for order in unpaid_orders:
+                  order.bill.add(next_bill)
+
+              print("Created bill {} for user {}".format(next_bill.uuid, user.username))
+
+        # We're done for this round :-)
         print("=> Done.")