docs: def _initialize_indices(model_class, name, bases, attrs):
      """Stores the list of indexed attributes."""
      model_class._indices = []
      for k, v in attrs.iteritems():
          if isinstance(v, (Attribute, ListField)) and v.indexed:
              model_class._indices.append(k)
      if model_class._meta['indices']:
          model_class._indices.extend(model_class._meta['indices'])