Prevent loops in many-to-many self-referencing relationship (Postgres) - postgresql

Question

Suppose you want to have a hierarchy of things where a thing can have multiple thing parents and children:
CREATE TABLE thing (
id SERIAL PRIMARY KEY
);
CREATE TABLE thing_association (
parent_id INTEGER NOT NULL,
child_id INTEGER NOT NULL,
PRIMARY KEY (parent_id, child_id),
CHECK (parent_id != child_id),
FOREIGN KEY (parent_id) REFERENCES thing(id),
FOREIGN KEY (child_id) REFERENCES thing(id)
);
The CHECK constraint prevents a thing from having a relationship with itself, and the PRIMARY KEY constraint prevents duplicate relationships, but can loops be prevented?
More precisely, if row (x, y) exists in the thing_association table, can the row (y, x) be prevented from being inserted?
Going a step further, if rows (x, y) and (y, z) exist in the thing_association, can the row (z, x) be prevented from being inserted?

Answer 1

I was hoping to accomplish this without triggers, but I'm not sure that's possible. I was able to accomplish this with a BEFORE INSERT trigger:
CREATE TABLE thing (
id SERIAL PRIMARY KEY
);
CREATE TABLE thing_association (
parent_id INTEGER NOT NULL,
child_id INTEGER NOT NULL,
PRIMARY KEY (parent_id, child_id),
CHECK (parent_id != child_id),
FOREIGN KEY (parent_id) REFERENCES thing(id),
FOREIGN KEY (child_id) REFERENCES thing(id)
);
/* maps every thing to all of it's parents */
CREATE VIEW thing_hierarchy AS
WITH RECURSIVE children AS (
SELECT
child_id,
parent_id
FROM thing_association
UNION SELECT
children.child_id,
parents.parent_id
FROM thing_association AS parents
INNER JOIN children
ON children.parent_id = parents.child_id
) SELECT * FROM children;
CREATE FUNCTION check_thing_association_loop() RETURNS TRIGGER AS $$
BEGIN
IF ((NEW.parent_id, NEW.child_id) in (SELECT child_id, parent_id FROM thing_hierarchy)) THEN
RAISE EXCEPTION 'Cannont create a hierarchy loop';
END IF;
RETURN NEW;
END;
$$ LANGUAGE plpgsql;
CREATE TRIGGER thing_association_insert_check
BEFORE INSERT ON thing_association
FOR EACH ROW EXECUTE FUNCTION check_thing_association_loop();
You could merge the view into the trigger function, but the view is useful on its own and it keeps things succint.