CREATE OR REPLACE FUNCTION function1() RETURNS trigger AS
$$
BEGIN
  SELECT a.field_1
    into new.field_1
  FROM relation_a a
  WHERE a.id_a = new.fk_a;

  RETURN new;
END
$$
LANGUAGE PLPGSQL;

DROP TRIGGER IF EXISTS trigger1 on relation_b;
CREATE TRIGGER trigger1
   BEFORE UPDATE OR INSERT
   ON relation_b
   FOR EACH ROW EXECUTE PROCEDURE function1();