How to copy rows into a new a one to many relationship - sql

Question

I'm trying to copy a set of data in a one to many relationship to create a new set of the same data in a new, but unrelated one to many relationship. Lets call them groups and items. Groups have a 1-* relation with items - one group has many items.
I've tried to create a CTE to do this, however I can't get the items inserted (in y) as the newly inserted groups don't have any items associated with them yet. I think I need to be able to access old. and new. like you would in a trigger, but I can't work out how to do this.
I think I could solve this by introducing a previous parent id into the templateitem table, or maybe a temp table with the data required to enable me to join on that, but I was wondering if it is possible to solve it this way?
SQL Fiddle Keeps Breaking on me, so I've put the code here as well:
DROP TABLE IF EXISTS meta.templateitem;
DROP TABLE IF EXISTS meta.templategroup;
CREATE TABLE meta.templategroup (
templategroup_id serial PRIMARY KEY,
groupname text,
roworder int
);
CREATE TABLE meta.templateitem (
templateitem_id serial PRIMARY KEY,
itemname text,
templategroup_id INTEGER NOT NULL REFERENCES meta.templategroup(templategroup_id)
);
INSERT INTO meta.templategroup (groupname, roworder) values ('Group1', 1), ('Group2', 2);
INSERT INTO meta.templateitem (itemname, templategroup_id) values ('Item1A',1), ('Item1B',1), ('Item2A',2);
WITH
x AS (
INSERT INTO meta.templategroup (groupname, roworder)
SELECT distinct groupname || '_v1' FROM meta.templategroup where templategroup_id in (1,2)
RETURNING groupname, templategroup_id, roworder
),
y AS (
Insert INTO meta.templateitem (itemname, templategroup_id)
Select itemname, x.templategroup_id
From meta.templateitem i
INNER JOIN x on x.templategroup_id = i.templategroup_id
RETURNING *
)
SELECT * FROM y;

Answer 1

Use an auxiliary column templategroup.old_id:
ALTER TABLE meta.templategroup ADD old_id int;
WITH x AS (
INSERT INTO meta.templategroup (groupname, roworder, old_id)
SELECT DISTINCT groupname || '_v1', roworder, templategroup_id
FROM meta.templategroup
WHERE templategroup_id IN (1,2)
RETURNING templategroup_id, old_id
),
y AS (
INSERT INTO meta.templateitem (itemname, templategroup_id)
SELECT itemname, x.templategroup_id
FROM meta.templateitem i
INNER JOIN x ON x.old_id = i.templategroup_id
RETURNING *
)
SELECT * FROM y;
templateitem_id | itemname | templategroup_id
-----------------+----------+------------------
4 | Item1A | 3
5 | Item1B | 3
6 | Item2A | 4
(3 rows)
It's impossible to do that in a single plain sql query without an additional column. You have to store the old ids somewhere. As an alternative you can use plpgsql and anonymous code block:
Before:
select *
from meta.templategroup
join meta.templateitem using (templategroup_id);
templategroup_id | groupname | roworder | templateitem_id | itemname
------------------+-----------+----------+-----------------+----------
1 | Group1 | 1 | 1 | Item1A
1 | Group1 | 1 | 2 | Item1B
2 | Group2 | 2 | 3 | Item2A
(3 rows)
Insert:
do $$
declare
grp record;
begin
for grp in
select distinct groupname || '_v1' groupname, roworder, templategroup_id
from meta.templategroup
where templategroup_id in (1,2)
loop
with insert_group as (
insert into meta.templategroup (groupname, roworder)
values (grp.groupname, grp.roworder)
returning templategroup_id
)
insert into meta.templateitem (itemname, templategroup_id)
select itemname || '_v1', g.templategroup_id
from meta.templateitem i
join insert_group g on grp.templategroup_id = i.templategroup_id;
end loop;
end $$;
After:
select *
from meta.templategroup
join meta.templateitem using (templategroup_id);
templategroup_id | groupname | roworder | templateitem_id | itemname
------------------+-----------+----------+-----------------+-----------
1 | Group1 | 1 | 1 | Item1A
1 | Group1 | 1 | 2 | Item1B
2 | Group2 | 2 | 3 | Item2A
3 | Group1_v1 | 1 | 4 | Item1A_v1
3 | Group1_v1 | 1 | 5 | Item1B_v1
4 | Group2_v1 | 2 | 6 | Item2A_v1
(6 rows)