I prefer to use CTE in postgres to do upserts, this is a non complete example, but you get the general idea:

  WITH upsert AS (
    UPDATE some_table 
    SET attrib = 'foo'
    WHERE id = 123
    RETURNING *
  ) INSERT INTO some_table ( id , attrib )
    SELECT 123 as id, 'foo' as attrib
    WHERE 123 NOT IN (SELECT id FROM upsert );