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postgresqldatabase-designforeign-keysconstraintsreferential-integrity

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

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?

Solution

  • 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. Related answer on dba.SE with detailed instructions:

    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 caveats. Best make that a NOT VALID constraint. See: