Fix unnecessary call to organisation API endpoint

We’re caching the organisation name, but still talking to the API
to see if the organisation exists.

`Service().organisation_id` only goes to the JSON for the service.

`Service().organisation` makes a separate API call.

We only need the former to know if a service belongs to an organisation.

tests/app/models/test_service.py

@@ -1,5 +1,6 @@
 import uuid
 
+from app.models.organisation import Organisation
 from app.models.service import Service
 from app.models.user import User
 from tests import organisation_json
@@ -195,3 +196,54 @@ def test_organisation_type_when_service_and_its_org_both_have_an_org_type(mocker
     mocker.patch('app.organisations_client.get_organisation', return_value=org)
 
     assert service.organisation_type == 'local'
+
+
+def test_organisation_name_comes_from_cache(mocker, service_one):
+    mock_redis_get = mocker.patch(
+        'app.extensions.RedisClient.get',
+        return_value=b'"Borchester Council"',
+    )
+    mock_get_organisation = mocker.patch('app.organisations_client.get_organisation')
+    service = Service(service_one)
+    service._dict['organisation'] = ORGANISATION_ID
+
+    assert service.organisation_name == 'Borchester Council'
+    mock_redis_get.assert_called_once_with(f'organisation-{ORGANISATION_ID}-name')
+    assert mock_get_organisation.called is False
+
+
+def test_organisation_name_goes_into_cache(mocker, service_one):
+    mocker.patch(
+        'app.extensions.RedisClient.get',
+        return_value=None,
+    )
+    mock_redis_set = mocker.patch(
+        'app.extensions.RedisClient.set',
+    )
+    mocker.patch(
+        'app.organisations_client.get_organisation',
+        return_value=organisation_json(),
+    )
+    service = Service(service_one)
+    service._dict['organisation'] = ORGANISATION_ID
+
+    assert service.organisation_name == 'Test Organisation'
+    mock_redis_set.assert_called_once_with(
+        f'organisation-{ORGANISATION_ID}-name',
+        '"Test Organisation"',
+        ex=604800,
+    )
+
+
+def test_service_without_organisation_doesnt_need_org_api(mocker, service_one):
+    mock_redis_get = mocker.patch('app.extensions.RedisClient.get')
+    mock_get_organisation = mocker.patch('app.organisations_client.get_organisation')
+    service = Service(service_one)
+    service._dict['organisation'] = None
+
+    assert service.organisation_id is None
+    assert service.organisation_name is None
+    assert isinstance(service.organisation, Organisation)
+
+    assert mock_redis_get.called is False
+    assert mock_get_organisation.called is False