Prevent and/or detect cycles in postgres

Question

Assuming a schema like the following:

CREATE TABLE node (
  id       SERIAL PRIMARY KEY,
  name     VARCHAR,
  parentid INT REFERENCES node(id)
);
         


        

Answer 1

To answer my own question, I came up with a trigger that prevents this:

CREATE OR REPLACE FUNCTION detect_cycle() RETURNS TRIGGER AS
$func$
DECLARE
  loops INTEGER;
BEGIN
   EXECUTE 'WITH RECURSIVE search_graph(id, parentid, name, depth, path, cycle) AS (
        SELECT g.id, g.parentid, g.name, 1,
          ARRAY[g.id],
          false
        FROM node g
      UNION ALL
        SELECT g.id, g.parentid, g.name, sg.depth + 1,
          path || g.id,
          g.id = ANY(path)
        FROM node g, search_graph sg
        WHERE g.id = sg.parentid AND NOT cycle
)
SELECT count(*) FROM search_graph where cycle = TRUE' INTO loops;
IF loops > 0 THEN
  RAISE EXCEPTION 'Loop detected!';
ELSE
  RETURN NEW;
END IF;
END
$func$ LANGUAGE plpgsql;

CREATE TRIGGER detect_cycle_after_update
AFTER UPDATE ON node
FOR EACH ROW EXECUTE PROCEDURE detect_cycle();

So, if you try to create a loop, like in the question:

UPDATE node SET parentid = 2 WHERE id = 1;

You get an EXCEPTION:

ERROR:  Loop detected!