Nova usage report fix

Gracefully handled case where rawdata entry does
not exist for request_id while generating nova usage
audit report.

Change-Id: I675b2b5e9c4be70d45fc2385f1b448c159610f56

reports/nova_usage_audit.py

@@ -95,10 +95,7 @@ def _get_exists(beginning, ending):
 
 def cell_and_compute(instance, launched_at):
     usage = InstanceUsage.find(instance, launched_at)[0]
-    try:
-        deployment = usage.latest_deployment_for_request_id()
-    except IndexError:
-        deployment = None
+    deployment = usage.latest_deployment_for_request_id()
     cell = (deployment and deployment.name) or '-'
     compute = usage.host() or '-'
     return cell, compute

stacktach/models.py

@@ -185,8 +185,10 @@ class InstanceUsage(models.Model):
         return raw and raw.deployment
 
     def latest_raw_for_request_id(self):
-        return self.request_id and RawData.objects.filter(
-            request_id=self.request_id).order_by('-id')[0]
+        raw = []
+        if self.request_id:
+            raw = RawData.objects.filter(request_id=self.request_id).order_by('-id')
+        return (len(raw) > 0 and raw[0]) or None
 
     def host(self):
         raw = self.latest_raw_for_request_id()