docs: def _load_base_class_fields(new_class, bases, attrs):
          """If this class is subclassing another Entity, add that Entity's
          fields.  Note that we loop over the bases in *reverse*.
          This is necessary in order to maintain the correct order of fields.
          """
          for base in reversed(bases):
              if hasattr(base, 'meta_') and \
                      hasattr(base.meta_, 'declared_fields'):
                  base_class_fields = {
                      field_name: field_obj for (field_name, field_obj)
                      in base.meta_.declared_fields.items()
                      if field_name not in attrs and not field_obj.identifier
                  }
                  new_class._load_fields(base_class_fields)