Update ft_billing DAO functions to use postage

* Updated the 'fetch_billing_data_for_day' DAO function to take postage into
account
* Updated the 'update_fact_billing' DAO function to insert postage for
new rows. When updating rows which are identical apart from the postage, the
original row will be kept. (This behaviour will change once postage is
added to the primary key - at this point, upserting will add a new row.)
* Also changed some fixtures / test set up functions to take postage
into account

tests/app/celery/test_reporting_tasks.py

@@ -210,6 +210,44 @@ def test_create_nightly_billing_letter(
     assert record.rate_multiplier == 2.0
 
 
+def test_create_nightly_billing_different_letter_postage(
+        notify_db_session,
+        sample_letter_template,
+        mocker):
+    yesterday = datetime.now() - timedelta(days=1)
+
+    mocker.patch('app.dao.fact_billing_dao.get_rate', side_effect=mocker_get_rate)
+
+    create_notification(
+        created_at=yesterday,
+        template=sample_letter_template,
+        status='delivered',
+        sent_by='dvla',
+        billable_units=2,
+        postage='first'
+    )
+    create_notification(
+        created_at=yesterday,
+        template=sample_letter_template,
+        status='delivered',
+        sent_by='dvla',
+        billable_units=2,
+        postage='second'
+    )
+
+    records = FactBilling.query.all()
+    assert len(records) == 0
+    # Celery expects the arguments to be a string or primitive type.
+    yesterday_str = datetime.strftime(yesterday, "%Y-%m-%d")
+    create_nightly_billing(yesterday_str)
+
+    # ft_billing rows will not get upserted since postage is not part of the primary key
+    record = FactBilling.query.one()
+    assert record.notification_type == LETTER_TYPE
+    assert record.bst_date == datetime.date(yesterday)
+    assert record.postage == 'first'
+
+
 def test_create_nightly_billing_null_sent_by_sms(
         sample_service,
         sample_template,