PL/pgSQL trigger to stop a river crossing another river - sql

Question

I have to write a trigger to stop a river crossing another river. I'm really struggling with it and any help would be appreciated. myriver is the table containing all the information on the rivers. So upon insert of a new river if it crosses an existing river, I should receive an error. Here's what I have:
CREATE FUNCTION river_check() RETURNS TRIGGER AS $river_check$
BEGIN
-- Check that gid is given
IF NEW.gid IS NULL THEN
RAISE EXCEPTION 'river gid cannot be null';
END IF;
NEW.the_geom = (SELECT r.the_geom FROM myriver as r
WHERE ST_CROSSES(NEW.the_geom, r.the_geom));
IF NEW.the_geom THEN
RAISE EXCEPTION 'a river cannot cross another river';
END IF;
RETURN NEW;
END;
$river_check$ LANGUAGE plpgsql;
-- Function river_check is linked to a TRIGGER of same name:
CREATE TRIGGER river_check
BEFORE INSERT OR UPDATE ON myriver
FOR EACH ROW EXECUTE PROCEDURE river_check();

Answer 1

You are using a column of the to-be-inserted/-updated row (NEW.the_geom) as a temporary variable. So you will either over-write that column variable (giving the new row a bogus value), or get an irrelevant result on your IF check (because NEW.the_geom had data in it anyway before the trigger was run).
Note also that Postgres, and pl/pgsql, is strictly typed, so you can't use an arbitrary value in an IF statement to see if it is "empty", like you would in a scripting language like PHP.
You need to either add a DECLARE block to give you a proper temporary variable, and check if it IS NULL; Or just use an EXISTS check directly:
IF EXISTS (
SELECT r.the_geom FROM myriver as r
WHERE ST_CROSSES(NEW.the_geom, r.the_geom)
)
THEN
RAISE EXCEPTION 'a river cannot cross another river';
END IF;