Insert into two referencing tables by selecting from a single table - postgresql

Question

I have 2 permanent tables in my PostgreSQL 12 database with a one-to-many relationship (thing, and thing_identifier). The second -- thing_identifier -- has a column referencing thing, such that thing_identifier can hold multiple, external identifiers for a given thing:
CREATE TABLE IF NOT EXISTS thing
(
thing_id SERIAL PRIMARY KEY,
thing_name TEXT, --this is not necessarily unique
thing_attribute TEXT --also not unique
);
CREATE TABLE IF NOT EXISTS thing_identifier
(
id SERIAL PRIMARY KEY,
thing_id integer references thing (thing_id),
identifier text
);
I need to insert some new data into thing and thing_identifier, both of which come from a table I created by using COPY to pull the contents of a large CSV file into the database, something like:
CREATE TABLE IF NOT EXISTS things_to_add
(
id SERIAL PRIMARY KEY,
guid TEXT, --a unique identifier used by the supplier
thing_name TEXT, --not unique
thing_attribute TEXT --also not unique
);
Sample data:
INSERT INTO things_to_add (guid, thing_name) VALUES
('[111-22-ABC]','Thing-a-ma-jig','pretty thing'),
('[999-88-XYZ]','Herk-a-ma-fob','blue thing');
The goal is to have each row in things_to_add result in one new row, each, in thing and thing_identifier, as in the following:
thing:
| thing_id | thing_name | thing attribute |
|----------|---------------------|-------------------|
| 1 | thing-a-ma-jig | pretty thing
| 2 | herk-a-ma-fob | blue thing
thing_identifier:
| id | thing_id | identifier |
|----|----------|------------------|
| 8 | 1 | '[111-22-ABC]' |
| 9 | 2 | '[999-88-XYZ]' |
I could use a CTE INSERTstatement (with RETURNING thing_id) to get the thing_id that results from the INSERT on thing, but I can't figure out how to get both that thing_id from the INSERT on thing and the original guid from things_to_add, which needs to go into thing_identifier.identifier.
Just to be clear, the only guaranteed unique column in thing is thing_id, and the only guaranteed unique column in things_to_add is id (which we don't want to store) and guid (which is what we want in thing_identifier.identifier), so there isn't any way to join thing and things_to_add after the INSERT on thing.

Answer 1

You can retrieve the thing_to_add.guid from a JOIN :
WITH list AS
(
INSERT INTO thing (thing_name)
SELECT thing_name
FROM things_to_add
RETURNING thing_id, thing_name
)
INSERT INTO thing_identifier (thing_id, identifier)
SELECT l.thing_id, t.guid
FROM list AS l
INNER JOIN thing_to_add AS t
ON l.thing_name = t.thing_name
Then, if thing.thing_name is not unique, the problem is more tricky. Updating both tables thing and thing_identifier from the same trigger on thing_to_add may solve the issue :
CREATE OR REPLACE FUNCTION after_insert_thing_to_add ()
RETURNS TRIGGER LANGUAGE sql AS
$$
WITH list AS
(
INSERT INTO thing (thing_name)
SELECT NEW.thing_name
RETURNING thing_id
)
INSERT INTO thing_identifier (thing_id, identifier)
SELECT l.thing_id, NEW.guid
FROM list AS l ;
$$
DROP TRIGGER IF EXISTS after_insert ON thing_to_add ;
CREATE TRIGGER after_insert
AFTER INSERT
ON thing_to_add
FOR EACH ROW
EXECUTE PROCEDURE after_insert_thing_to_add ();