Constraint, based on join with another table - postgresql

Question

I have table tariffs, with two columns: (tariff_id, reception)
I have table users, with two columns: (user_id, reception)
And I have table users_tariffs with two columns: (user_id, tariff_id).
I want to prevent situation when tariff from one reception is assigned to user from another reception. How can I do that?
E.G
Users:
user_id | reception
Putin | Russia
Trump | USA
Tariffs:
tariff_id | reception
cheap | USA
expensive | Russia
Wrong situation at users_tariffs, because Cheap tariff is for USA only:
user_id | tariff_id
Putin | Cheap

Answer 1

SOLUTION 1: FOREIGN KEY CONSTRAINTS
I am assuming the following table definitions.
In particular, the composite key in user_tariffs makes this a many-to-many relationship between users and tariffs.
CREATE TABLE tariffs (tariff_id int NOT NULL PRIMARY KEY,
reception text NOT NULL);
CREATE TABLE users (user_id int NOT NULL PRIMARY KEY,
reception text NOT NULL);
CREATE TABLE user_tariffs (tariff_id int NOT NULL REFERENCES tariffs (tariff_id),
user_id int NOT NULL REFERENCES users (user_id),
PRIMARY KEY (tariff_id, user_id));
You probably need a combination of all three columns somewhere, so let's create this:
ALTER TABLE user_tariffs ADD COLUMN reception text;
UPDATE user_tariffs a
SET reception = b.reception
FROM (SELECT * FROM tariffs) b
WHERE a.tariff_id = b.tariff_id;
ALTER TABLE user_tariffs ALTER COLUMN reception SET NOT NULL;
Now we can use FOREIGN KEY REFERENCES (user_id, reception) into users.
CREATE UNIQUE INDEX ON tariffs (tariff_id, reception);
ALTER TABLE user_tariffs ADD FOREIGN KEY (tariff_id, reception)
REFERENCES tariffs (tariff_id, reception);
In addition, we can use FK REFs (tariff_id, reception) into tariffs.
CREATE UNIQUE INDEX ON users (user_id, reception);
ALTER TABLE user_tariffs ADD FOREIGN KEY (user_id, reception)
REFERENCES users (user_id, reception);
Populate with data:
INSERT INTO users VALUES (1, 'cheap'), (2, 'expensive');
INSERT INTO tariffs VALUES (1, 'cheap'), (2, 'expensive');
Now assume we have the following data (user_id, tariff_id) to insert:
WITH data (user_id, tariff_id)
AS (VALUES (1, 2), (2, 1)), -- here is your application data
datas (user_id, tariff_id, reception)
AS (SELECT user_id,
tariff_id,
(SELECT u.reception -- reception calculated by user
FROM users u
WHERE u.user_id = d.user_id)
FROM data d)
INSERT INTO user_tariffs SELECT * FROM datas ;
Then you cannot insert the data, because you can only add (1, 1) or (2, 2) with the same reception, but not (1, 2) or (2, 1) with different reception's. The error message is:
ERROR: insert or update on table "user_tariffs" violates foreign key constraint "user_tariffs_user_id_fkey1"
DETAIL: Key (user_id, reception)=(2, cheap) is not present in table "users".
But you can insert with data AS VALUES (1, 1), (2, 2).
I think the FOREIGN KEY CONSTRAINT solution is to be preferred.
Please describe your functional dependencies, if you want better table designs.
SOLUTION 2: TRIGGER
-- DROP TABLE user_tariffs CASCADE;
-- DROP TABLE users CASCADE;
-- DROP TABLE tariffs CASCADE;
CREATE TABLE tariffs (tariff_id int NOT NULL PRIMARY KEY,
reception text NOT NULL);
CREATE TABLE users (user_id int NOT NULL PRIMARY KEY,
reception text NOT NULL);
CREATE TABLE user_tariffs (tariff_id int NOT NULL REFERENCES tariffs (tariff_id),
user_id int NOT NULL REFERENCES users (user_id),
PRIMARY KEY (tariff_id, user_id));
INSERT INTO users VALUES (1, 'cheap'), (2, 'expensive');
INSERT INTO tariffs VALUES (1, 'cheap'), (2, 'expensive');
-- table user_tariffs (user_id, tariff_id) only, without reception column.
Create a function with return type trigger:
CREATE OR REPLACE FUNCTION check_reception()
RETURNS trigger AS $$
DECLARE valid boolean := false;
BEGIN
SELECT (SELECT u.reception FROM users u WHERE u.user_id = NEW.user_id)
= (SELECT t.reception FROM tariffs t WHERE t.tariff_id = NEW.tariff_id)
INTO valid FROM user_tariffs ;
IF valid = false
THEN RAISE EXCEPTION '(user, tariff, reception) invalid.';
END IF;
RETURN NEW;
END; $$ LANGUAGE plpgsql ;
and register it:
CREATE TRIGGER reception_trigger
AFTER INSERT OR UPDATE ON user_tariffs
FOR EACH ROW EXECUTE PROCEDURE check_reception();
Now try to insert (1, 2), which would be (cheap, expensive) and is not allowed:
INSERT INTO user_tariffs VALUES (1, 2);
ERROR: (user, tariff, reception) invalid.
KONTEXT: PL/pgSQL function check_reception() line 7 at RAISE
But we can insert (1, 1), which is (cheap, cheap) without problem:
INSERT INTO user_tariffs VALUES (1, 1);
SELECT * FROM user_tariffs;
Remark
Triggers are not the best solution here, in my opinion. Try to avoid triggers, if possible. They can have side effects (transactions etc). Check StackOverflow for further details :)