CREATE FUNCTION check_book_permission_before_checkout()
  returns TRIGGER AS $$
DECLARE
  requires_permission BOOLEAN;
BEGIN
  requires_permission := (SELECT requires_permission FROM books WHERE id = NEW.book_id);
  IF requires_permission AND NEW.permitted_by_id IS NULL
  THEN RAISE EXCEPTION 'A permission is required to check out the requested book.';
  END IF;
END;
$$
language plpgsql;

CREATE TRIGGER validate_book_checkout_policy
  before INSERT
  ON book_lendings
  FOR each ROW EXECUTE PROCEDURE check_book_permission_before_checkout();