In order to correctly select the rate for letters you need the billable_unit or page count to use to match on the letter_rates.

A new column is added to the query to populate ft_billing which is only populated for letter.

app/dao/fact_billing_dao.py

@@ -149,6 +149,11 @@ def fetch_billing_data_for_day(process_day, service_id=None):
                       ).label('sent_by'),
         func.coalesce(table.rate_multiplier, 1).cast(Integer).label('rate_multiplier'),
         func.coalesce(table.international, False).label('international'),
+        case(
+            [
+                (table.notification_type == 'letter', table.billable_units),
+            ]
+        ).label('letter_page_count'),
         func.sum(table.billable_units).label('billable_units'),
         func.count().label('notifications_sent'),
         Service.crown,
@@ -162,6 +167,7 @@ def fetch_billing_data_for_day(process_day, service_id=None):
         table.service_id,
         table.notification_type,
         'sent_by',
+        'letter_page_count',
         table.rate_multiplier,
         table.international,
         Service.crown
@@ -182,9 +188,9 @@ def get_rates_for_billing():
     return non_letter_rates, letter_rates
 
 
-def get_rate(non_letter_rates, letter_rates, notification_type, date, crown=None, rate_multiplier=None):
+def get_rate(non_letter_rates, letter_rates, notification_type, date, crown=None, letter_page_count=None):
     if notification_type == LETTER_TYPE:
-        return next(r[3] for r in letter_rates if date > r[0] and crown == r[1] and rate_multiplier == r[2])
+        return next(r[3] for r in letter_rates if date > r[0] and crown == r[1] and letter_page_count == r[2])
     elif notification_type == SMS_TYPE:
         return next(r[2] for r in non_letter_rates if notification_type == r[0] and date > r[1])
     else:
@@ -198,9 +204,8 @@ def update_fact_billing(data, process_day):
                     data.notification_type,
                     process_day,
                     data.crown,
-                    data.rate_multiplier)
+                    data.letter_page_count)
     billing_record = create_billing_record(data, rate, process_day)
-
     table = FactBilling.__table__
     '''
        This uses the Postgres upsert to avoid race conditions when two threads try to insert

tests/app/dao/test_ft_billing_dao.py

@@ -47,6 +47,11 @@ def set_up_yearly_data():
                                   template=letter_template,
                                   notification_type='letter',
                                   rate=0.33)
+                create_ft_billing(bst_date='{}-{}-{}'.format(year, mon, d),
+                                  service=service,
+                                  template=letter_template,
+                                  notification_type='letter',
+                                  rate=0.30)
     return service
 
 
@@ -236,7 +241,7 @@ def test_get_rate(notify_db_session):
     letter_rate = get_rate(non_letter_rates=non_letter_rates, letter_rates=letter_rates,
                            notification_type='letter',
                            crown=True,
-                           rate_multiplier=1,
+                           letter_page_count=1,
                            date=datetime.utcnow())
 
     assert rate == 2.2
@@ -302,7 +307,7 @@ def test_fetch_monthly_billing_for_year_return_financial_year(notify_db_session)
     # returns 3 rows, per month, returns financial year april to end of march
     # Orders by Month
 
-    assert len(results) == 36
+    assert len(results) == 48
     assert str(results[0].month) == "2016-04-01"
     assert results[0].notification_type == 'email'
     assert results[0].notifications_sent == 30
@@ -312,21 +317,26 @@ def test_fetch_monthly_billing_for_year_return_financial_year(notify_db_session)
     assert results[1].notification_type == 'letter'
     assert results[1].notifications_sent == 30
     assert results[1].billable_units == 30
-    assert results[1].rate == Decimal('0.33')
+    assert results[1].rate == Decimal('0.30')
-    assert str(results[2].month) == "2016-04-01"
+    assert str(results[1].month) == "2016-04-01"
-    assert results[2].notification_type == 'sms'
+    assert results[2].notification_type == 'letter'
     assert results[2].notifications_sent == 30
     assert results[2].billable_units == 30
-    assert results[2].rate == Decimal('0.162')
+    assert results[2].rate == Decimal('0.33')
-    assert str(results[3].month) == "2016-05-01"
+    assert str(results[3].month) == "2016-04-01"
-    assert str(results[35].month) == "2017-03-01"
+    assert results[3].notification_type == 'sms'
+    assert results[3].notifications_sent == 30
+    assert results[3].billable_units == 30
+    assert results[3].rate == Decimal('0.162')
+    assert str(results[4].month) == "2016-05-01"
+    assert str(results[47].month) == "2017-03-01"
 
 
 def test_fetch_billing_totals_for_year(notify_db_session):
     service = set_up_yearly_data()
     results = fetch_billing_totals_for_year(service_id=service.id, year=2016)
 
-    assert len(results) == 3
+    assert len(results) == 4
     assert results[0].notification_type == 'email'
     assert results[0].notifications_sent == 365
     assert results[0].billable_units == 365
@@ -335,9 +345,14 @@ def test_fetch_billing_totals_for_year(notify_db_session):
     assert results[1].notification_type == 'letter'
     assert results[1].notifications_sent == 365
     assert results[1].billable_units == 365
-    assert results[1].rate == Decimal('0.33')
+    assert results[1].rate == Decimal('0.3')
 
-    assert results[2].notification_type == 'sms'
+    assert results[2].notification_type == 'letter'
     assert results[2].notifications_sent == 365
     assert results[2].billable_units == 365
-    assert results[2].rate == Decimal('0.162')
+    assert results[2].rate == Decimal('0.33')
+
+    assert results[3].notification_type == 'sms'
+    assert results[3].notifications_sent == 365
+    assert results[3].billable_units == 365
+    assert results[3].rate == Decimal('0.162')