Creating a SQL trigger to update tables when entering data in a view - postgresql

Question

If I have this setup:
CREATE TABLE category(
category_id serial PRIMARY KEY,
category_name text UNIQUE NOT NULL -- must be UNIQUE
);
CREATE TABLE parts (
part_id serial PRIMARY KEY,
category_id int REFERENCES product,
part_name text
);
CREATE VIEW partview AS
SELECT com.part_id, cat.category_name, com.part_name
FROM parts com
LEFT JOIN category cat USING (category_id);
How do I create a trigger so that when I insert data into the view, the source tables are updated?
I tried this... but it doesn't work :(
CREATE FUNCTION insert_view_func()
RETURNS trigger as
$func$
BEGIN
INSERT INTO parts (category_name)
select (select category_id from category where category_name = category.category_name)
RETURNING category_id as id
into new.componentid;
return new;
END
$func$ language plpgsql;
create trigger insert_view_trig
INSTEAD of insert on partview
for each row execute procedure insert_view_func();

Answer 1

The big issue with view insert triggers on non-simple views is you do not know what is be inserted.
In this case that could be Category or Parts or both. Your trigger has to handle both. Here that is not much a issue here:
create or replace function insert_view_func()
returns trigger
language plpgsql
as $$
begin
insert into category (category_name) values(new.category_name)
on conflict do nothing;
insert into parts (category_id, part_name)
select category_id, new.part_name
from category
where category_name = new.category_name;
return new;
end ;
$$ ;
That, however, is not the major issue here. Your data model setup a 1:M relationship between Category:Parts.
Not a problem if that is what you really want, but it does open a potential problem. Since the Part_Name is not unique it opens
the possibility of multiple parts with the same name (see fiddle), but each associated to a separate category.
This could become quite confusing. To avoid this you may want to consider a M:M relationship and creating a resolution table. The other option would be modifying the trigger function to check for existing part_name. Better yet make Part_Name unique.