Add a helper method to Image to parse ancestor string.

data/database.py

@@ -616,6 +616,12 @@ class Image(BaseModel):
       (('security_indexed_engine', 'security_indexed'), False),
     )
 
+  def ancestor_id_list(self):
+    """ Returns an integer list of ancestor ids, ordered chronologically from
+        root to direct parent.
+    """
+    return map(int, self.ancestors.split('/')[1:-1])
+
 
 _ImageProxy.initialize(Image)