Split out handling of BST queries for reuse

app/dao/notifications_dao.py

@@ -27,6 +27,7 @@ from app.models import (
 
 from app.dao.dao_utils import transactional
 from app.statsd_decorators import statsd
+from app.utils import get_london_month_from_utc_column
 
 
 def dao_get_notification_statistics_for_service_and_day(service_id, day):
@@ -225,16 +226,8 @@ def get_notifications_for_job(service_id, job_id, filter_dict=None, page=1, page
 def get_notification_billable_unit_count_per_month(service_id, year):
     start, end = get_financial_year(year)
 
-    """
+    month = get_london_month_from_utc_column(NotificationHistory.created_at)
-     The query needs to sum the billable_units per month, but this needs to be the month in BST (British Standard Time).
+
-     The database stores all timestamps as UTC without the timezone.
-      - First set the timezone on created_at to UTC
-      - then convert the timezone to BST (or Europe/London)
-      - lastly truncate the datetime to month to group the sum of the billable_units
-    """
-    month = func.date_trunc("month",
-                            func.timezone("Europe/London", func.timezone("UTC",
-                                                                         NotificationHistory.created_at)))
     notifications = db.session.query(
         month,
         func.sum(NotificationHistory.billable_units)

app/dao/services_dao.py

@@ -30,6 +30,7 @@ from app.models import (
     TEMPLATE_TYPES,
 )
 from app.statsd_decorators import statsd
+from app.utils import get_london_month_from_utc_column
 
 
 def dao_fetch_all_services(only_active=False):
@@ -247,13 +248,7 @@ def dao_fetch_monthly_historical_stats_for_service(service_id, year):
     monday_of_notification_week = func.date_trunc('week', NotificationHistory.created_at).label('week_start')
     start, end = get_financial_year(year)
 
-    month = func.date_trunc(
+    month = get_london_month_from_utc_column(NotificationHistory.created_at)
-        "month",
-        func.timezone(
-            "Europe/London",
-            func.timezone("UTC", NotificationHistory.created_at)
-        )
-    )
 
     rows = db.session.query(
         NotificationHistory.notification_type,

app/utils.py

@@ -2,6 +2,7 @@ from datetime import datetime, timedelta
 
 import pytz
 from flask import url_for
+from sqlalchemy import func
 from notifications_utils.template import SMSMessageTemplate, PlainTextEmailTemplate
 
 
@@ -46,3 +47,19 @@ def get_london_midnight_in_utc(date):
 def get_midnight_for_day_before(date):
     day_before = date - timedelta(1)
     return get_london_midnight_in_utc(day_before)
+
+
+def get_london_month_from_utc_column(column):
+    """
+     Where queries need to count notifications by month it needs to be
+     the month in BST (British Summer Time).
+     The database stores all timestamps as UTC without the timezone.
+      - First set the timezone on created_at to UTC
+      - then convert the timezone to BST (or Europe/London)
+      - lastly truncate the datetime to month with which we can group
+        queries
+    """
+    return func.date_trunc(
+        "month",
+        func.timezone("Europe/London", func.timezone("UTC", column))
+    )