docs: def before_insert(mapper, conn, target):
          """event.listen method for Sqlalchemy to set the sequence for this
          object and create an ObjectNumber value for the id_"""
          from sqlalchemy import text
  
          if not target.id:
              sql = text('SELECT max(f_id)+1 FROM files WHERE f_d_vid = :did')
  
              target.id, = conn.execute(sql, did=target.d_vid).fetchone()
  
              if not target.id:
                  target.id = 1
  
          if target.contents and isinstance(target.contents, six.text_type):
              target.contents = target.contents.encode('utf-8')
  
          File.before_update(mapper, conn, target)