Triggers: using table's primary key value to update another field

I'm trying to write a generic function that will use the primary key field
value to insert/update another field in the same table (see code below).

What I'm having trouble with is figuring out how to grab the primary key
value of the current row. I tried tacking on a var with the pkey row name to
NEW, but that doesn't work (didn't think it would).

Is there a way to do this?

CREATE FUNCTION add_parent_id () RETURNS TRIGGER AS '
DECLARE
pkey TEXT;
BEGIN
SELECT INTO pkey column_name
FROM information_schema.constraint_column_usage
WHERE table_name = TG_RELNAME
AND constraint_name ILIKE ''%_pkey'';

IF NEW.parent_id IS NULL THEN
SELECT NEW.pkey INTO NEW.parent_id;
END IF;

RETURN NEW;
END;
' LANGUAGE 'plpgsql';

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Randall Perry
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