docs: def each_fields(for_fields, fields):
      """
      select_related(
          'field__related_field1__text1', 'field__related_field1__text2',
          'field__related_field2__text1', 'field__related_field2__text2',
      )
      select_related(*each_fields(['field__related_field1', 'field__related_field2'], ['text1', 'text2']))
      each_fields('field__related_field1', ['text1', 'text2'])
      """
      if isinstance(for_fields, basestring):
          for_fields = (for_fields,)
      r = set()
      for ff in for_fields:
          for f in fields:
              r.add(ff + '__' + f)
      return list(r)