WIP revamped bill logic

uncloud/uncloud_pay/helpers.py

@@ -1,7 +1,10 @@
 from functools import reduce
+from datetime import datetime
 from rest_framework import mixins
 from rest_framework.viewsets import GenericViewSet
-from .models import Bill, Payment, PaymentMethod
+from django.db.models import Q
+from .models import Bill, Payment, PaymentMethod, Order
+from django.utils import timezone
 
 def sum_amounts(entries):
     return reduce(lambda acc, entry: acc + entry.amount, entries, 0)
@@ -20,6 +23,52 @@ def get_payment_method_for(user):
 
     return None
 
+def beginning_of_month(date):
+    return datetime(year=date.year, date=now.month, day=0)
+
+def generate_bills_for(year, month, user, allowed_delay):
+    # /!\ We exclusively work on the specified year and month.
+
+    # Default values for next bill (if any). Only saved at the end of
+    # this method, if relevant.
+    tz = timezone.get_current_timezone()
+    next_bill = Bill(owner=user,
+            starting_date=datetime(year=year, month=month, day=1, tzinfo=tz),
+            ending_date=datetime(year=year, month=month, day=28, tzinfo=tz),
+            creation_date=timezone.now(),
+            due_date=timezone.now() + allowed_delay)
+
+    # Select all orders active on the request period.
+    orders = Order.objects.filter(
+            Q(ending_date__gt=next_bill.starting_date) | Q(ending_date__isnull=True),
+            owner=user)
+
+    # Check if there is already a bill covering the order and period pair:
+    #   * Get latest bill by ending_date: previous_bill.ending_date
+    #   * If previous_bill.ending_date is before next_bill.ending_date, a new
+    #   bill has to be generated.
+    unpaid_orders = []
+    for order in orders:
+        try:
+            previous_bill = order.bill.latest('-ending_date')
+        except ObjectDoesNotExist:
+            previous_bill = None
+
+        if previous_bill == None or previous_bill.ending_date < next_bill.ending_date:
+            unpaid_orders.append(order)
+
+    # Commit next_bill if it there are 'unpaid' orders.
+    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)
+
+        return next_bill
+
+    # Return None if no bill was created.
 
 class ProductViewSet(mixins.CreateModelMixin,
                    mixins.RetrieveModelMixin,