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}.

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]