Trigger

trigger.plsql

CREATE OR REPLACE FUNCTION check_group() RETURNS TRIGGER AS
$BODY$
  DECLARE
    group_count integer;
    musician_count integer;
  BEGIN
    SELECT COUNT(*) FROM artist
      WHERE artist.id = NEW."groupId" AND is_group = TRUE
      LIMIT 1
      INTO group_count;

    IF (group_count = 0) THEN
        RAISE EXCEPTION 'Please, use group for membership record';
        RETURN OLD;
    END IF;

    SELECT COUNT(*) FROM artist
      WHERE artist.id = NEW."musicianId" AND is_group = FALSE
      LIMIT 1
      INTO musician_count;

    IF (musician_count = 0) THEN
        RAISE EXCEPTION 'Please, use musician for membership record';
        RETURN OLD;
    END IF;

    RETURN NEW;
  END;
$BODY$ LANGUAGE plpgsql;

CREATE TRIGGER check_group
  BEFORE INSERT OR UPDATE ON membership
  FOR EACH ROW EXECUTE PROCEDURE check_group();