diff --git a/nova/objects/resource_class_cache.py b/nova/objects/resource_class_cache.py
index 244b743..dcbd50c 100644
--- a/nova/objects/resource_class_cache.py
+++ b/nova/objects/resource_class_cache.py
@@ -12,6 +12,7 @@

 import sqlalchemy as sa

+from nova.db.sqlalchemy import api as db_api
 from nova.db.sqlalchemy import api_models as models
 from nova.objects import fields

@@ -31,7 +32,9 @@ class ResourceClassCache(object):
         self.id_cache = {}
         self.str_cache = {}

-    def id_from_string(self, rc_str):
+    @db_api.api_context_manager.reader
+    @staticmethod
+    def _id_from_string(ctx, rc_str):
         """Given a string representation of a resource class -- e.g. "DISK_GB"
         or "IRON_SILVER" -- return the integer code for the resource class. For
         standard resource classes, this integer code will match the list of
@@ -65,6 +68,9 @@ class ResourceClassCache(object):
         self.id_cache[rc_str] = rc_id
         return rc_id

+    def id_from_string(self, rc_str):
+        return self._id_from_string(self.ctx, rc_str)
+
     def string_from_id(self, rc_id):
         """The reverse of the id_from_string() method. Given a supplied numeric
         identifier for a resource class, we look up the corresponding string