Test for letters not returned recently

If you have some returned letters, but none in the last 7 days, we don’t
count how many you have in the last 7 days. But we should test to make
sure we’re not going to the database again.

tests/app/service/test_rest.py

@@ -3396,6 +3396,30 @@ def test_get_returned_letter_statistics(admin_request, sample_service):
     }
 
 
+@freeze_time('2019-12-11 13:30')
+def test_get_returned_letter_statistics_with_old_returned_letters(
+    mocker,
+    admin_request,
+    sample_service,
+):
+    create_returned_letter(sample_service, reported_at=datetime.utcnow() - timedelta(days=8))
+    create_returned_letter(sample_service, reported_at=datetime.utcnow() - timedelta(days=800))
+
+    count_mock = mocker.patch(
+        'app.service.rest.fetch_recent_returned_letter_count',
+    )
+
+    assert admin_request.get(
+        'service.returned_letter_statistics',
+        service_id=sample_service.id,
+    ) == {
+        'returned_letter_count': 0,
+        'most_recent_report': '2019-12-03 00:00:00.000000',
+    }
+
+    assert count_mock.called is False
+
+
 def test_get_returned_letter_statistics_with_no_returned_letters(
     mocker,
     admin_request,