Add "cost" field to monthly usage API

This starts to replace the calculation in Admin [^1] and, similar
to the yearly API, also correctly attributes free allowance when
we have a rate change during a month.

[^1]: 474d7dfda8/app/templates/views/usage.html (L98)

app/billing/billing_schemas.py

@@ -22,6 +22,7 @@ def serialize_ft_billing_remove_emails(rows):
             "chargeable_units": row.chargeable_units,
             "rate": float(row.rate),
             "postage": row.postage,
+            "cost": float(row.cost),
         }
         for row in rows
         if row.notification_type != 'email'

app/dao/fact_billing_dao.py

@@ -244,6 +244,7 @@ def fetch_monthly_billing_for_year(service_id, year):
                 query.c.rate.label("rate"),
                 query.c.postage.label("postage"),
                 query.c.notification_type.label("notification_type"),
+                func.sum(query.c.cost).label("cost"),
             ).group_by(
                 query.c.rate,
                 query.c.notification_type,

tests/app/billing/test_rest.py

@@ -175,6 +175,7 @@ def test_get_yearly_usage_by_monthly_from_ft_billing(admin_request, notify_db_se
     assert letter_row["chargeable_units"] == 30
     assert letter_row["rate"] == 0.33
     assert letter_row["postage"] == "second"
+    assert letter_row["cost"] == 9.9
 
     assert sms_row["month"] == "April"
     assert sms_row["notification_type"] == "sms"
@@ -182,6 +183,8 @@ def test_get_yearly_usage_by_monthly_from_ft_billing(admin_request, notify_db_se
     assert sms_row["chargeable_units"] == 30
     assert sms_row["rate"] == 0.162
     assert sms_row["postage"] == "none"
+    # free allowance is 4, so (30 - 4) * 0.162
+    assert sms_row["cost"] == 4.212
 
 
 def set_up_yearly_data():

tests/app/dao/test_ft_billing_dao.py

@@ -428,6 +428,7 @@ def test_fetch_monthly_billing_for_year(notify_db_session):
     assert results[0].billable_units == 30
     assert results[0].chargeable_units == 30
     assert results[0].rate == Decimal('0')
+    assert results[0].cost == Decimal('0')
 
     assert str(results[1].month) == "2016-04-01"
     assert results[1].notification_type == 'letter'
@@ -435,6 +436,7 @@ def test_fetch_monthly_billing_for_year(notify_db_session):
     assert results[1].billable_units == 30
     assert results[1].chargeable_units == 30
     assert results[1].rate == Decimal('0.30')
+    assert results[1].cost == Decimal('9')
 
     assert str(results[1].month) == "2016-04-01"
     assert results[2].notification_type == 'letter'
@@ -442,6 +444,7 @@ def test_fetch_monthly_billing_for_year(notify_db_session):
     assert results[2].billable_units == 30
     assert results[2].chargeable_units == 30
     assert results[2].rate == Decimal('0.33')
+    assert results[2].cost == Decimal('9.9')
 
     assert str(results[3].month) == "2016-04-01"
     assert results[3].notification_type == 'sms'
@@ -449,6 +452,8 @@ def test_fetch_monthly_billing_for_year(notify_db_session):
     assert results[3].billable_units == 30
     assert results[3].chargeable_units == 30
     assert results[3].rate == Decimal('0.162')
+    # free allowance is 10, so (30 - 10) * 0.162
+    assert results[3].cost == Decimal('3.24')
 
     assert str(results[4].month) == "2016-05-01"
     assert str(results[47].month) == "2017-03-01"
@@ -468,6 +473,7 @@ def test_fetch_monthly_billing_for_year_variable_rates(notify_db_session):
     assert results[0].billable_units == 1
     assert results[0].chargeable_units == 1
     assert results[0].rate == Decimal('0.33')
+    assert results[0].cost == Decimal('0.33')
 
     assert str(results[1].month) == "2018-05-01"
     assert results[1].notification_type == 'letter'
@@ -475,6 +481,7 @@ def test_fetch_monthly_billing_for_year_variable_rates(notify_db_session):
     assert results[1].billable_units == 1
     assert results[1].chargeable_units == 1
     assert results[1].rate == Decimal('0.36')
+    assert results[1].cost == Decimal('0.36')
 
     assert str(results[2].month) == "2018-05-01"
     assert results[2].notification_type == 'sms'
@@ -482,6 +489,8 @@ def test_fetch_monthly_billing_for_year_variable_rates(notify_db_session):
     assert results[2].billable_units == 4
     assert results[2].chargeable_units == 4
     assert results[2].rate == Decimal('0.015')
+    # 4 free units sent on the 16th, 0 on the 17th
+    assert results[2].cost == Decimal('0')
 
     assert str(results[3].month) == "2018-05-01"
     assert results[3].notification_type == 'sms'
@@ -489,6 +498,8 @@ def test_fetch_monthly_billing_for_year_variable_rates(notify_db_session):
     assert results[3].billable_units == 5
     assert results[3].chargeable_units == 5
     assert results[3].rate == Decimal('0.162')
+    # 1 free unit on the 16th, 1 on the 17th (+ 3 paid)
+    assert results[3].cost == Decimal('0.486')
 
 
 @freeze_time('2018-08-01 13:30:00')