Set fields used to group query results.

        :type value: str or sequence of strings
        :param value: Each value is a string giving the name of a
                      property to use to group results together.

def distinct_on(self, value):
        """Set fields used to group query results.

        :type value: str or sequence of strings
        :param value: Each value is a string giving the name of a
                      property to use to group results together.
        """
        if isinstance(value, str):
            value = [value]
        self._distinct_on[:] = value