docs: def dump_relation(api, rel_cfg, pid, data):
      """Dump a specific relation to a data dict."""
      schema_class = rel_cfg.schema
      if schema_class is not None:
          schema = schema_class()
          schema.context['pid'] = pid
          result, errors = schema.dump(api)
          data.setdefault(rel_cfg.name, []).append(result)