Added Scheduled task to get stats for template usage

Currently some pages are timing out due to the time it takes to perform
database queries. This is an attempt to improve the performance by
performing the query against the notification history table once a day
and use the notification table for a delta between midnight and the when
the page is run and combine the results.

- Added Celery task for doing the work
- Added a dao to handle the insert and update of the stats table
- Updated tests to test the new functionality

app/dao/services_dao.py

@@ -1,7 +1,7 @@
 import uuid
-from datetime import date, datetime, timedelta
+from datetime import date, datetime, timedelta, time
 
-from sqlalchemy import asc, func
+from sqlalchemy import asc, func, extract
 from sqlalchemy.orm import joinedload
 from flask import current_app
 
@@ -40,7 +40,7 @@ from app.models import (
 )
 from app.service.statistics import format_monthly_template_notification_stats
 from app.statsd_decorators import statsd
-from app.utils import get_london_month_from_utc_column, get_london_midnight_in_utc
+from app.utils import get_london_month_from_utc_column, get_london_midnight_in_utc, get_london_year_from_utc_column
 from app.dao.annual_billing_dao import dao_insert_annual_billing
 
 DEFAULT_SERVICE_PERMISSIONS = [
@@ -520,3 +520,25 @@ def dao_fetch_active_users_for_service(service_id):
     )
 
     return query.all()
+
+
+@statsd(namespace="dao")
+def dao_fetch_monthly_historical_stats_by_template():
+    month = get_london_month_from_utc_column(NotificationHistory.created_at)
+    year = get_london_year_from_utc_column(NotificationHistory.created_at)
+    end_date = datetime.combine(date.today(), time.min)
+
+    return db.session.query(
+        NotificationHistory.template_id,
+        month.label('month'),
+        year.label('year'),
+        func.count().label('count')
+    ).filter(
+        NotificationHistory.created_at < end_date
+    ).group_by(
+        NotificationHistory.template_id,
+        month,
+        year
+    ).order_by(
+        NotificationHistory.template_id
+    ).all()

app/dao/stats_template_usage_by_month_dao.py

@@ -0,0 +1,24 @@
+from app import db
+from app.models import StatsTemplateUsageByMonth
+
+
+def insert_or_update_stats_for_template(template_id, month, year, count):
+    result = db.session.query(
+        StatsTemplateUsageByMonth
+    ).filter(
+        StatsTemplateUsageByMonth.template_id == template_id,
+        StatsTemplateUsageByMonth.month == month,
+        StatsTemplateUsageByMonth.year == year
+    ).update(
+        {
+            'count': count
+        }
+    )
+    if result == 0:
+        new_sms_sender = StatsTemplateUsageByMonth(
+            template_id=template_id,
+            month=month,
+            year=year,
+            count=count
+        )
+        db.session.add(new_sms_sender)