fix(overage-billing): use relative delta and months, instead of days (#5060)

Author: matthewelwell

api/organisations/tasks.py

@@ -3,6 +3,7 @@
 from datetime import timedelta
 
 from app_analytics.influxdb_wrapper import get_current_api_usage
+from dateutil.relativedelta import relativedelta
 from django.conf import settings
 from django.core.mail import send_mail
 from django.db.models import F, Max, Q
@@ -154,7 +155,7 @@ def charge_for_api_call_count_overages():
 
     # Get the period where we're interested in any new API usage
     # notifications for the relevant billing period (ie, this month).
-    api_usage_notified_at = now - timedelta(days=30)
+    api_usage_notified_at = now - relativedelta(months=1)
 
     # Since we're only interested in monthly billed accounts, set a wide
     # threshold to catch as many billing periods that could be roughly

api/tests/unit/organisations/test_unit_organisations_tasks.py

@@ -5,6 +5,7 @@
 
 import pytest
 from core.helpers import get_current_site_url
+from dateutil.relativedelta import relativedelta
 from django.core.mail.message import EmailMultiAlternatives
 from django.template.loader import render_to_string
 from django.utils import timezone
@@ -834,10 +835,19 @@ def test_charge_for_api_call_count_overages_scale_up(
     organisation.subscription.subscription_id = "fancy_sub_id23"
     organisation.subscription.plan = "scale-up-v2"
     organisation.subscription.save()
+
+    # In order to cover an edge case found in production use, we make the
+    # notification date just outside the previous 30 days, because we want
+    # to make sure that we cover the case where someone with very high usage
+    # is notified in the first day of their subscription period (in a 31-day
+    # month).
+    notification_date = now - (timedelta(days=30) + timedelta(minutes=30))
+    assert notification_date > now - relativedelta(months=1)
+
     OrganisationAPIUsageNotification.objects.create(
         organisation=organisation,
         percent_usage=100,
-        notified_at=now,
+        notified_at=notification_date,
     )
 
     mocker.patch("organisations.chargebee.chargebee.chargebee.Subscription.retrieve")