Property to be used when saving a custom field into
        :class:`couchbasekit.document.Document` instance.

        :returns: The value to be saved for the field within
            :class:`couchbasekit.document.Document` instances.
        :rtype: mixed

def value(self):
        """Property to be used when saving a custom field into
        :class:`couchbasekit.document.Document` instance.

        :returns: The value to be saved for the field within
            :class:`couchbasekit.document.Document` instances.
        :rtype: mixed
        """
        if self._value is None:
            raise ValueError("%s's 'value' is not set." % type(self).__name__)
        return self._value