docs: def getClassKey(self, klass, key):
          """
          Return an instance based on klass/key.
  
          If an instance cannot be found then C{KeyError} is raised.
  
          @param klass: The class of the instance.
          @param key: The primary_key of the instance.
          @return: The instance linked to the C{klass}/C{key}.
          @rtype: Instance of C{klass}.
          """
          d = self._getClass(klass)
  
          return d[key]