triggers and functions trouble - postgresql

Question

I'm currently going through the growing pains of trying to learn about functions and triggers. I'm trying to do a problem from a book I'm reading , but i dont understand how to do certain parts.
using this table
create table movies (
id integer primary key,
title varchar(255) not null,
year integer
);
insert into movies values (1, 'The Croods', 2013);
insert into movies values (2, 'Now You See Me', 2013);
insert into movies values (3, 'Argo', 2012);
insert into movies values (4, 'Jurassic World', 2015);
create table discs (
id integer primary key,
movie_id integer not null references movies(id),
type_id integer references disc_types(id),
price decimal(10,2),
available boolean
);
insert into discs values (1, 1, 1, 1.59, 't');
insert into discs values (2, 1, 1, 1.59, 'f');
insert into discs values (3, 1, 2, 2.99, 'f');
insert into discs values (4, 2, 1, 1.29, 't');
insert into discs values (5, 2, 1, 1.29, 't');
insert into discs values (6, 2, 2, 2.99, 't');
insert into discs values (7, 3, 2, 2.59, 't');
insert into discs values (8, 3, 2, 2.59, 't');
create table customers (
id integer primary key,
name varchar(255),
email varchar(255)
);
insert into customers values (1, 'John', 'john#hotmail.com');
insert into customers values (2, 'Jane', 'jane#gmail.com');
create table rentals (
id integer primary key,
customer_id integer not null references customers(id),
disc_id integer not null references discs(id),
date_rented date,
date_returned date
);
insert into rentals values (1, 1, 7, '2013-10-01', '2013-10-03');
insert into rentals values (2, 2, 5, '2013-10-05', '2013-10-06');
insert into rentals values (3, 2, 2, '2013-11-02', null);
insert into rentals values (4, 2, 3, '2013-11-02', null);
create table ratings (
customer_id integer not null references customers(id),
movie_id integer not null references movies(id),
rating integer,
primary key (customer_id, movie_id)
);
insert into ratings values (1, 1, 1);
insert into ratings values (1, 2, 4);
insert into ratings values (1, 3, 5);
insert into ratings values (2, 1, 4);
my logic was that i would have the new values of the ratings table that were going to be inserted or updated and use those to compare to whats in the rentals table to see if that customer had rented that movie already, if they did then they could enter a rating. but i cant transfer that logic in this lol. unless there an easier way to do this.

Answer 1

The loop inside the function complicates matters a bit, let's see if we can get rid of it. Your ratings table has a reference to customer and movie so we need a join.
SELECT COUNT(*) INTO rented FROM rentals WHERE disc_id IN
(SELECT id from discs INNER JOIN
rentals ON disc_id = discs.id where movie_id = new.movie_id)
AND customer_id = new.customer_id
Right this should make the logic of your stored procedure a lot easier. I am now leaving you to finish it because this after all is a learning exercise.
You need this sort of a join because it's more efficient and simpler than the loop. The ratings table has a reference to the movie_id but the rentals table only has a disc_id thus to find out if the user has rented a particular movie, you need to join it through the disc table.
You will need to change the return values. ref: http://www.postgresql.org/docs/9.2/static/plpgsql-trigger.html
Row-level triggers fired BEFORE can return null to signal the trigger
manager to skip the rest of the operation for this row (i.e.,
subsequent triggers are not fired, and the INSERT/UPDATE/DELETE does
not occur for this row). If a nonnull value is returned then the
operation proceeds with that row value
And also note that you do not do an INSERT inside your trigger function. You just return a non null value for the insert to proceed.

Answer 2

This is the EXISTS() version. (BTW: the definition for movies is missing)
CREATE OR REPLACE FUNCTION rate_only_rented()
RETURNS TRIGGER AS $func$
BEGIN
IF ( NOT EXISTS (
SELECT *
FROM rentals r
JOIN discs d ON r.disc_id = d.id
WHERE d.movie_id = NEW.movie_id
AND r.customer_id = NEW.customer_id
) ) THEN
RAISE EXCEPTION 'you(%) have not rented this movie(%) before'
, NEW.customer_id ,NEW.movie_id;
RETURN NULL;
ELSE
RETURN NEW;
END IF;
END;
$func$ language plpgsql;
And the trigger:
CREATE TRIGGER rate_only_rented
AFTER INSERT OR UPDATE
ON ratings
FOR EACH ROW
EXECUTE PROCEDURE rate_only_rented()
;