fix(config): when rolling a new config copy from the latest release instead of latest config for the app

This helps mitigate deis#798 for day to day use (quick fix) as the previous config is no longer pulled from the top of the application config stack and instead pulls the latest release config.
Doing that will prevent the logic from re-using a potentially broken Config object. When a Release object is deleted an Config object can be orphaned but still available.

Author: helgi

rootfs/api/models/config.py

@@ -2,6 +2,7 @@
 from django.db import models
 from jsonfield import JSONField
 
+from api.models.release import Release
 from api.models import UuidAuditedModel, DeisException
 
 
@@ -110,7 +111,14 @@ def set_tags(self, previous_config):
     def save(self, **kwargs):
         """merge the old config with the new"""
         try:
-            previous_config = self.app.config_set.latest()
+            # Get config from the latest available release
+            try:
+                previous_config = self.app.release_set.latest().config
+            except Release.DoesNotExist:
+                # If that doesn't exist then fallback on app config
+                # usually means a totally new app
+                previous_config = self.app.config_set.latest()
+
             for attr in ['cpu', 'memory', 'tags', 'registry', 'values']:
                 data = getattr(previous_config, attr, {}).copy()
                 new_data = getattr(self, attr, {}).copy()