code: def _prepare_source(selector, source):
      
      tablename, fields = get_data_specifier(selector)
      if len(fields) != 1:
          raise ItsdbError(
              'Selector must specify exactly one data column: {}'
              .format(selector)
          )
      if isinstance(source, TestSuite):
          if not tablename:
              tablename = source.relations.find(fields[0])[0]
          source = source[tablename]
      cols = list(source.fields.keys()) + fields
      return source, cols
docs: Normalize source rows and selectors.