CONSTRAINT to check values from a remotely related table (via join etc.)

Question

I would like to add a constraint that will check values from related table.

I have 3 tables:

CREATE TABLE somethink_usr_rel (
    user_id BIGINT NOT NULL,
    stomethink_id BIGINT NOT NULL
);

CREATE TABLE usr (
    id BIGINT NOT NULL,
    role_id BIGINT NOT NULL
);

CREATE TABLE role (
    id BIGINT NOT NULL,
    type BIGINT NOT NULL
);

(If you want me to put constraint with FK let me know.)

I want to add a constraint to somethink_usr_rel that checks type in role ("two tables away"), e.g.:

ALTER TABLE somethink_usr_rel
    ADD CONSTRAINT CH_sm_usr_type_check 
    CHECK (usr.role.type = 'SOME_ENUM');

I tried to do this with JOINs but didn't succeed. Any idea how to achieve it?

Answer 1

CHECK constraints cannot currently reference other tables. The manual:

Currently, CHECK expressions cannot contain subqueries nor refer to variables other than columns of the current row.

One way is to use a trigger like demonstrated by @Wolph.

A clean solution without triggers: add redundant columns and include them in FOREIGN KEY constraints, which are the first choice to enforce referential integrity anyway. Related answer on dba.SE with detailed instructions:

• Enforcing constraints “two tables away”

Another option would be to "fake" an IMMUTABLE function doing the check and use that in a CHECK constraint. Postgres will allow this, but be aware of possible consequences. You best make that a NOT VALID constraint. Details:

• Disable all constraints and table checks while restoring a dump

Answer 2

A CHECK constraint is not an option if you need joins. You can create a trigger which raises an error instead.

Have a look at this example: http://www.postgresql.org/docs/9.1/static/plpgsql-trigger.html#PLPGSQL-TRIGGER-EXAMPLE

CREATE TABLE emp (
    empname text,
    salary integer,
    last_date timestamp,
    last_user text
);

CREATE FUNCTION emp_stamp() RETURNS trigger AS $emp_stamp$
    BEGIN
        -- Check that empname and salary are given
        IF NEW.empname IS NULL THEN
            RAISE EXCEPTION 'empname cannot be null';
        END IF;
        IF NEW.salary IS NULL THEN
            RAISE EXCEPTION '% cannot have null salary', NEW.empname;
        END IF;

        -- Who works for us when she must pay for it?
        IF NEW.salary < 0 THEN
            RAISE EXCEPTION '% cannot have a negative salary', NEW.empname;
        END IF;

        -- Remember who changed the payroll when
        NEW.last_date := current_timestamp;
        NEW.last_user := current_user;
        RETURN NEW;
    END;
$emp_stamp$ LANGUAGE plpgsql;

CREATE TRIGGER emp_stamp BEFORE INSERT OR UPDATE ON emp
    FOR EACH ROW EXECUTE PROCEDURE emp_stamp();

Answer 3

...i did it so (nazwa=user name, firma = company name) :

CREATE TABLE users
(
  id bigserial  CONSTRAINT firstkey PRIMARY KEY,
  nazwa character varying(20),
  firma character varying(50)
);


CREATE TABLE test
(
  id bigserial  CONSTRAINT firstkey PRIMARY KEY,
  firma character varying(50),
  towar character varying(20),
  nazwisko character varying(20)
);

ALTER TABLE public.test ENABLE ROW LEVEL SECURITY;

CREATE OR REPLACE FUNCTION whoIAM3() RETURNS varchar(50) as $$
declare
    result varchar(50);
   BEGIN
 select into result users.firma from users where users.nazwa = current_user;
    return result;
    END;

    $$ LANGUAGE plpgsql;


CREATE POLICY user_policy ON public.test
    USING (firma = whoIAM3());

CREATE FUNCTION test_trigger_function()
RETURNS trigger AS $$
BEGIN
  NEW.firma:=whoIam3();
return NEW;
END
$$ LANGUAGE 'plpgsql'
CREATE TRIGGER test_trigger_insert BEFORE INSERT  ON test FOR EACH ROW EXECUTE PROCEDURE  test_trigger_function();