Add a ref_test key to Anomaly entities and surface it in alerts JSON.

dashboard/dashboard/models/anomaly.py

Index: dashboard/dashboard/models/anomaly.py
diff --git a/dashboard/dashboard/models/anomaly.py b/dashboard/dashboard/models/anomaly.py
index bc3e296855b78c2e0f19bff0f4d18eec85389c75..f1323398922fc9246533f4d11d47c3de048d88bc 100644
--- a/dashboard/dashboard/models/anomaly.py
+++ b/dashboard/dashboard/models/anomaly.py
@@ -8,6 +8,7 @@ import sys
 
 from google.appengine.ext import ndb
 
+from dashboard.common import utils
 from dashboard.models import alert
 
 # A string to describe the magnitude of a change from zero to non-zero.
@@ -55,6 +56,9 @@ class Anomaly(alert.Alert):
   # is a corresponding step up later on, or vice versa.)
   recovered = ndb.BooleanProperty(indexed=True, default=False)
 
+  # If the TestMetadata alerted upon has a ref build, store the ref build.
+  ref_test = ndb.KeyProperty(indexed=False)
+
   # The corresponding units from the TestMetaData entity.
   units = ndb.StringProperty(indexed=False)
 
@@ -92,6 +96,11 @@ class Anomaly(alert.Alert):
     else:
       return '%f' % abs(self.absolute_delta)
 
+  def GetRefTestPath(self):
+    if not self.ref_test:
+      return None
+    return utils.TestPath(self.ref_test)
+
   def SetIsImprovement(self, test=None):
     """Sets whether the alert is an improvement for the given test."""
     if not test: