code: def _validate_derived_from(cursor, model):
      
      derived_from_uri = model.metadata.get('derived_from_uri')
      if derived_from_uri is None:
          return  # bail out early
  
      # Can we parse the value?
      try:
          ident_hash = parse_archive_uri(derived_from_uri)
          uuid_, version = split_ident_hash(ident_hash, split_version=True)
      except (ValueError, IdentHashSyntaxError, IdentHashShortId) as exc:
          raise exceptions.InvalidMetadata('derived_from_uri', derived_from_uri,
                                           original_exception=exc)
      # Is the ident-hash a valid pointer?
      args = [uuid_]
      table = 'modules'
      version_condition = ''
      if version != (None, None,):
          args.extend(version)
          table = 'modules'
          version_condition = " AND major_version = %s" \
                              " AND minor_version {} %s" \
                              .format(version[1] is None and 'is' or '=')
      cursor.execute("""SELECT 't' FROM {} WHERE uuid::text = %s{}"""
                     .format(table, version_condition), args)
      try:
          _exists = cursor.fetchone()[0]  # noqa
      except TypeError:  # None type
          raise exceptions.InvalidMetadata('derived_from_uri', derived_from_uri)
  
      # Assign the derived_from value so that we don't have to split it again.
      model.metadata['derived_from'] = ident_hash
docs: Given a database cursor and model, check the derived-from
      value accurately points to content in the archive.
      The value can be nothing or must point to existing content.