code: public Storage<S> storageDelegate(StorableIndex<S> index) {
          if (mAllIndexInfoMap.get(index) instanceof ManagedIndex) {
              // Index is managed by this storage, which is typical.
              return null;
          }
          // Index is managed by master storage, most likely a primary key index.
          return mMasterStorage;
      }
docs: Required by StorageAccess.