INSERT… RETURNING for copying records

Good Afternoon,

I'm attempting to write a function that will duplicate a few records, but the catch is I need to have a mapping of the original pk to the new pk. I know I can use the RETURNING clause to get the new ids... but how to map that to the original ones is escaping me.

< Setup >

CREATE TABLE testing (rid serial PRIMARY KEY, category text NOT NULL, name text NOT NULL, fk_parent int4);

INSERT INTO testing (category, name, fk_parent) VALUES ('cat1', 'one', NULL), ('cat1', 'one.one', 1), ('cat1', 'one.two', 1);

SELECT * FROM testing;
+-----+----------+---------+-----------+
| rid | category | name | fk_parent |
+-----+----------+---------+-----------+
| 1 | cat1 | one | NULL |
| 2 | cat1 | one.one | 1 |
| 3 | cat1 | one.two | 1 |
+-----+----------+---------+-----------+

< Duplicating the records >

INSERT INTO testing (category, name, fk_parent) (select category, name, fk_parent from testing where category='cat1') returning rid, category, name, fk_parent;
+-----+----------+---------+-----------+
| rid | category | name | fk_parent |
+-----+----------+---------+-----------+
| 4 | cat1 | one | NULL |
| 5 | cat1 | one.one | 1 |
| 6 | cat1 | one.two | 1 |
+-----+----------+---------+-----------+

< What I'm looking for >
+--------------+-----+
| original_rid | rid |
+--------------+-----+
| 1 | 4 |
| 2 | 5 |
| 3 | 6 |
+--------------+-----+

< This doesn't work >

INSERT INTO testing (category, name, fk_parent) select category, name, fk_parent from testing as original where category='cat1' returning rid, category, name, fk_parent, original.rid;

Specifically, my goal is to be able to duplicate a subset of records and map any referenced foreign keys to the new ones from the copies. I could write a pl/pgsql function to loop through the records and build the mapping as I go, but I was thinking there might be a better way. Any thoughts?

Thanks!
Michael