How to insert new rows to a junction table Postgres - node.js

Question

I have a many to many relationship set up with with services and service_categories. Each has a table, and there is a third table to handle to relationship (junction table) called service_service_categories. I have created them like this:
CREATE TABLE services(
service_id SERIAL,
name VARCHAR(255),
summary VARCHAR(255),
profileImage VARCHAR(255),
userAgeGroup VARCHAR(255),
userType TEXT,
additionalNeeds TEXT[],
experience TEXT,
location POINT,
price NUMERIC,
PRIMARY KEY (id),
UNIQUE (name)
);
CREATE TABLE service_categories(
service_category_id SERIAL,
name TEXT,
description VARCHAR(255),
PRIMARY KEY (id),
UNIQUE (name)
);
CREATE TABLE service_service_categories(
service_id INT NOT NULL,
service_category_id INT NOT NULL,
PRIMARY KEY (service_id, service_category_id),
FOREIGN KEY (service_id) REFERENCES services(service_id) ON UPDATE CASCADE,
FOREIGN KEY (service_category_id) REFERENCES service_categories(service_category_id) ON UPDATE CASCADE
);
Now, in my application I would like to add a service_category to a service from a select list for example, at the same time as I create or update a service. In my node js I have this post route set up:
// Create a service
router.post('/', async( req, res) => {
try {
console.log(req.body);
const { name, summary } = req.body;
const newService = await pool.query(
'INSERT INTO services(name,summary) VALUES($1,$2) RETURNING *',
[name, summary]
);
res.json(newService);
} catch (err) {
console.log(err.message);
}
})
How should I change this code to also add a row to the service_service_categories table, when the new service ahas not been created yet, so has no serial number created?
If any one could talk me through the approach for this I would be grateful.
Thanks.

Answer 1

You can do this in the database by adding a trigger to the services table to insert a row into the service_service_categories that fires on row insert. The "NEW" keyword in the trigger function represents the row that was just inserted, so you can access the serial ID value.
https://www.postgresqltutorial.com/postgresql-triggers/
Something like this:
CREATE TRIGGER insert_new_service_trigger
AFTER INSERT
ON services
FOR EACH ROW
EXECUTE PROCEDURE insert_new_service();
Then your trigger function looks something like this (noting that the trigger function needs to be created before the trigger itself):
CREATE OR REPLACE FUNCTION insert_new_service()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$$
BEGIN
-- check to see if service_id has been created
IF NEW.service_id NOT IN (SELECT service_id FROM service_service_categories) THEN
INSERT INTO service_service_categories(service_id)
VALUES(NEW.service_id);
END IF;
RETURN NEW;
END;
$$;
However in your example data structure, it doesn't seem like there's a good way to link the service_categories.service_category_id serial value to this new row - you may need to change it a bit to accommodate

Answer 2

I managed to get it working to a point with multiple inserts and changing the schema a bit on services table. In the service table I added a column: category_id INT:
ALTER TABLE services
ADD COLUMN category_id INT;
Then in my node query I did this and it worked:
const newService = await pool.query(
`
with ins1 AS
(
INSERT INTO services (name,summary,category_id)
VALUES ($1,$2,$3) RETURNING service_id, category_id
),
ins2 AS
(
INSERT INTO service_service_categories (service_id,service_category_id) SELECT service_id, category_id FROM ins1
)
select * from ins1
`,
[name, summary, category_id]
);
Ideally I want to have multiple categories so the category_id column on service table, would become category_ids INT[]. and it would be an array of ids.
How would I put the second insert into a foreach (interger in the array), so it creates a new service_service_categories row for each id in the array?