def _ad_as_ts(expr):
 return (None if (expr is None) else bz.transform(expr, **{TS_FIELD_NAME: expr[AD_FIELD_NAME]}))

def _ad_as_ts ( expr ) : return ( None if ( expr is None ) else bz . transform ( expr , ** { TS_FIELD_NAME : expr [ AD_FIELD_NAME ] } ) )
duplicate the asof_date column as the timestamp column .