def single(self):
        """
        Return the associated node.

        :return: node
        """
        nodes = super(One, self).all()
        if nodes:
            if len(nodes) == 1:
                return nodes[0]
            else:
                raise CardinalityViolation(self, len(nodes))
        else:
            raise CardinalityViolation(self, 'none')
Return the associated node.

        :return: node