Only aggregate status when necessary for a service

This takes a similar approach to the nightly deletion task so that
we only create sub-tasks when there are actually notifications to
aggregate for a given type and day [1].

We're making this change to stop the duplication errors we're getting
at the moment and ensure the task can scale to more messages and more
services. There are two parts to this:

- Each subtask should now run within the 5 minute visibility timeout.
However, they may still be duplicated if the parent task overruns [2].

- The parent task creates a mininal number of subtasks, and the query
to determine this is very fast for a normal process day (milliseconds).

Since all tasks will run quickly, there should be no more duplication.

In order to test this more nuanced task, I rewrote the tests:

- One test checks the subtask is called correctly.
- One test checks we create all the right subtasks.

[1]: https://github.com/alphagov/notifications-api/pull/3381
[2]: https://docs.google.com/document/d/1MaP6Nyy3nJKkuh_4lP1wuDm19X8LZITOLRd9n3Ax-xg/edit#heading=h.q3intzwqhfzl

tests/app/celery/test_reporting_tasks.py

@@ -1,4 +1,3 @@
-import itertools
 from datetime import date, datetime, time, timedelta
 from decimal import Decimal
 from uuid import UUID
@@ -20,6 +19,7 @@ from app.models import (
     KEY_TYPE_TEAM,
     KEY_TYPE_TEST,
     LETTER_TYPE,
+    NOTIFICATION_TYPES,
     SMS_TYPE,
     FactBilling,
     FactNotificationStatus,
@@ -61,39 +61,58 @@ def test_create_nightly_billing_triggers_tasks_for_days(notify_api, mocker, day_
         assert mock_celery.apply_async.call_args_list[i][1]['kwargs'] == {'process_day': expected_kwargs[i]}
 
 
-@freeze_time('2019-08-01')
-def test_create_nightly_notification_status_triggers_tasks(notify_api, sample_service, mocker):
-    mock_celery = mocker.patch('app.celery.reporting_tasks.create_nightly_notification_status_for_service_and_day')
+@freeze_time('2019-08-01T00:30')
+def test_create_nightly_notification_status_triggers_tasks(
+    notify_api,
+    sample_service,
+    sample_template,
+    mocker,
+):
+    mock_celery = mocker.patch(
+        'app.celery.reporting_tasks.create_nightly_notification_status_for_service_and_day'
+    ).apply_async
+
+    create_notification(template=sample_template, created_at='2019-07-31')
     create_nightly_notification_status()
 
-    assert mock_celery.apply_async.call_count == (
-        (4 * 3)  # four days, three notification types
-        +
-        6  # six more days of just letters
+    mock_celery.assert_called_with(
+        kwargs={
+            'service_id': sample_service.id,
+            'process_day': '2019-07-31',
+            'notification_type': SMS_TYPE
+        },
+        queue=QueueNames.REPORTING
     )
 
-    for process_date, notification_type in itertools.product(
-        ['2019-07-31', '2019-07-30', '2019-07-29', '2019-07-28'],
-        [SMS_TYPE, EMAIL_TYPE, LETTER_TYPE]
-    ):
-        mock_celery.apply_async.assert_any_call(
-            kwargs={
-                'process_day': process_date,
-                'notification_type': notification_type,
-                'service_id': sample_service.id,
-            },
-            queue=QueueNames.REPORTING
-        )
 
-    for process_date in ['2019-07-27', '2019-07-26', '2019-07-25', '2019-07-24', '2019-07-23', '2019-07-22']:
-        mock_celery.apply_async.assert_any_call(
-            kwargs={
-                'process_day': process_date,
-                'notification_type': LETTER_TYPE,
-                'service_id': sample_service.id,
-            },
-            queue=QueueNames.REPORTING
-        )
+@freeze_time('2019-08-01T00:30')
+@pytest.mark.parametrize('notification_date, expected_types_aggregated', [
+    ('2019-08-01', set()),
+    ('2019-07-31', {EMAIL_TYPE, SMS_TYPE, LETTER_TYPE}),
+    ('2019-07-28', {EMAIL_TYPE, SMS_TYPE, LETTER_TYPE}),
+    ('2019-07-27', {LETTER_TYPE}),
+    ('2019-07-22', {LETTER_TYPE}),
+    ('2019-07-21', set()),
+])
+def test_create_nightly_notification_status_triggers_relevant_tasks(
+    notify_api,
+    sample_service,
+    mocker,
+    notification_date,
+    expected_types_aggregated,
+):
+    mock_celery = mocker.patch(
+        'app.celery.reporting_tasks.create_nightly_notification_status_for_service_and_day'
+    ).apply_async
+
+    for notification_type in NOTIFICATION_TYPES:
+        template = create_template(sample_service, template_type=notification_type)
+        create_notification(template=template, created_at=notification_date)
+
+    create_nightly_notification_status()
+
+    types = {call.kwargs['kwargs']['notification_type'] for call in mock_celery.mock_calls}
+    assert types == expected_types_aggregated
 
 
 @pytest.mark.parametrize('second_rate, records_num, billable_units, multiplier',