Prevent creation of too many associated rows in postgresql trigger.

row_max_association.sql

CREATE TRIGGER "Q_limit_answers_per_question" AFTER INSERT or UPDATE on question_answer 
  FOR EACH ROW EXECUTE PROCEDURE question_answer_insert();

CREATE OR REPLACE FUNCTION question_answer_insert()
RETURNS TRIGGER AS
$body$
BEGIN
    -- Lock the existing rows to prevent race condition with other txns.
    PERFORM * FROM question_answer WHERE question_id = NEW.question_id for UPDATE;
    -- Only allow insert/update if doesn't violate the constraints.
    IF (SELECT count(id) FROM question_answer where question_id = NEW.question_id) > 4
    THEN 
        RAISE EXCEPTION 'Too many rows for question %', NEW.question_id;
    END IF;
    RETURN NEW;
END;
$body$
LANGUAGE plpgsql;