code: private void cacheLinkedObject(String owningObjID,
                                     String linkFieldName,
                                     DBObject linkedObject) {
          // Find or create map for the owning object.
          Map<String, Map<String, DBObject>> objMap = m_linkedObjectMap.get(owningObjID);
          if (objMap == null) {
              objMap = new HashMap<String, Map<String, DBObject>>();
              m_linkedObjectMap.put(owningObjID, objMap);
          }
          
          // Find or create map for the link field.
          Map<String, DBObject> linkMap = objMap.get(linkFieldName);
          if (linkMap == null) {
              linkMap = new HashMap<String, DBObject>();
              objMap.put(linkFieldName, linkMap);
          }
          
          // Add the object to the link map.
          linkMap.put(linkedObject.getObjectID(), linkedObject);
      }
docs: link field name.