Add .cost() to notification model

In the V2 API, the GET response for an individual notification
returns a 'cost' value, which we can get by multiplying the
billable units by the per-message rate of the supplier who
sent the message.
Any notifications with billable units > 0 but without a
corresponding `ProviderRates` entry will blow up the application,
so make sure you've got one.

tests/app/conftest.py

@@ -29,6 +29,7 @@ from app.dao.api_key_dao import save_model_api_key
 from app.dao.jobs_dao import dao_create_job
 from app.dao.notifications_dao import dao_create_notification
 from app.dao.invited_user_dao import save_invited_user
+from app.dao.provider_rates_dao import create_provider_rates
 from app.clients.sms.firetext import FiretextClient
 
 
@@ -409,7 +410,8 @@ def sample_notification(notify_db,
                         create=True,
                         personalisation=None,
                         api_key_id=None,
-                        key_type=KEY_TYPE_NORMAL):
+                        key_type=KEY_TYPE_NORMAL,
+                        sent_by=None):
     if created_at is None:
         created_at = datetime.utcnow()
     if service is None:
@@ -441,7 +443,8 @@ def sample_notification(notify_db,
         'personalisation': personalisation,
         'notification_type': template.template_type,
         'api_key_id': api_key_id,
-        'key_type': key_type
+        'key_type': key_type,
+        'sent_by': sent_by
     }
     if job_row_number:
         data['job_row_number'] = job_row_number
@@ -841,3 +844,12 @@ def sample_service_whitelist(notify_db, notify_db_session, service=None, email_a
     notify_db.session.add(whitelisted_user)
     notify_db.session.commit()
     return whitelisted_user
+
+
+@pytest.fixture(scope='function')
+def sample_provider_rate(notify_db, notify_db_session, valid_from=None, rate=None, provider_identifier=None):
+    create_provider_rates(
+        provider_identifier=provider_identifier if provider_identifier is not None else 'mmg',
+        valid_from=valid_from if valid_from is not None else datetime.utcnow(),
+        rate=rate if rate is not None else 1,
+    )

tests/app/test_model.py

@@ -1,9 +1,7 @@
-import uuid
+import pytest
 from datetime import datetime
 
-import pytest
-
-from app import DATETIME_FORMAT
+from tests.app.conftest import sample_notification, sample_provider_rate
 from app.models import (
     Notification,
     ServiceWhitelist,
@@ -37,3 +35,29 @@ def test_should_build_service_whitelist_from_email_address(email_address):
 def test_should_not_build_service_whitelist_from_invalid_contact(recipient_type, contact):
     with pytest.raises(ValueError):
         ServiceWhitelist.from_string('service_id', recipient_type, contact)
+
+
+@pytest.mark.parametrize('provider, billable_units, expected_cost', [
+    ('mmg', 1, 3.5),
+    ('firetext', 2, 5),
+    ('ses', 0, 0)
+])
+def test_calculate_cost_from_notification_billable_units(
+        notify_db, notify_db_session, provider, billable_units, expected_cost
+):
+    provider_rates = [
+        ('mmg', datetime(2016, 7, 1), 1.5),
+        ('firetext', datetime(2016, 7, 1), 2.5),
+        ('mmg', datetime.utcnow(), 3.5),
+    ]
+    for provider_identifier, valid_from, rate in provider_rates:
+        sample_provider_rate(
+            notify_db,
+            notify_db_session,
+            provider_identifier=provider_identifier,
+            valid_from=valid_from,
+            rate=rate
+        )
+
+    notification = sample_notification(notify_db, notify_db_session, billable_units=billable_units, sent_by=provider)
+    assert notification.cost() == expected_cost

tests/app/v2/notifications/test_notification_schemas.py

@@ -3,7 +3,6 @@ import uuid
 import pytest
 from flask import json
 from jsonschema import ValidationError
-from notifications_utils.recipients import InvalidPhoneError, InvalidEmailError
 
 from app.v2.notifications.notification_schemas import post_sms_request, post_sms_response, post_email_request, \
     post_email_response