'Parses a filter arg in the format:
<colname>__<op>
:returns: colname, op tuple'

def _parse_filter_arg(self, arg):

    statement = arg.rsplit('__', 1)
    if (len(statement) == 1):
        return (arg, None)
    elif (len(statement) == 2):
        return ((statement[0], statement[1]) if (arg != 'pk__token') else (arg, None))
    else:
        raise QueryException("Can't    parse    '{0}'".format(arg))
