CREATE OR REPLACE FUNCTION insert_mp(
  _name VARCHAR,
  _description VARCHAR,
  _abbreviation VARCHAR,
  _price_without_taxes double precision,
  _um_nombre VARCHAR
) RETURNS INTEGER AS $$
DECLARE
  um_id INTEGER;
BEGIN
  /* poner varios if else por cada foreign key */
  IF EXISTS(select 1 from measure_units where abbreviation=_um_nombre) THEN
    um_id := (select measure_unit_id from measure_units where abbreviation=_um_nombre);
   ELSE
    RETURN 1;
  END IF;

  INSERT INTO material_types(name, description, abbreviation, price_without_taxes, measure_unit_id)
  VALUES (_name, _description, _abbreviation, _price_without_taxes, um_id);
  RETURN 0;
END
$$ language plpgsql;

--select insert_mp('bbb', 'ccc', 'ddd', 15.4, 'kg');