Function AddMetadataFromDots

r/src/table.cpp:219–275

}

arrow::Status AddMetadataFromDots(SEXP lst, int num_fields,
                                  std::shared_ptr<arrow::Schema>& schema) {
  // Preallocate the r_metadata object: list(attributes=list(), columns=namedList(fields))

  cpp11::writable::list metadata(2);
  metadata.names() = arrow::r::data::names_metadata;

  bool has_top_level_metadata = false;

  // "top level" attributes, only relevant if the first object is not named and a data
  // frame
  cpp11::strings names = Rf_getAttrib(lst, R_NamesSymbol);
  if (names[0] == "" && Rf_inherits(VECTOR_ELT(lst, 0), "data.frame") &&
      Rf_xlength(lst) == 1) {
    SEXP top_level = metadata[0] = arrow_attributes(VECTOR_ELT(lst, 0), true);
    if (!Rf_isNull(top_level) && XLENGTH(top_level) > 0) {
      has_top_level_metadata = true;
    }
  }

  // recurse to get all columns metadata
  cpp11::writable::list metadata_columns = CollectColumnMetadata(lst, num_fields);

  // Remove metadata for ExtensionType columns, because these have their own mechanism for
  // preserving R type information
  for (int i = 0; i < schema->num_fields(); i++) {
    if (schema->field(i)->type()->id() == Type::EXTENSION) {
      metadata_columns[i] = R_NilValue;
    }
  }

  // If all metadata_columns are NULL and there is no top-level metadata, set has_metadata
  // to false
  bool has_metadata = has_top_level_metadata;
  for (R_xlen_t i = 0; i < metadata_columns.size(); i++) {
    if (metadata_columns[i] != R_NilValue) {
      has_metadata = true;
      break;
    }
  }

  // Assign to the output metadata
  metadata[1] = metadata_columns;

  if (has_metadata) {
    SEXP serialise_call =
        PROTECT(Rf_lang2(arrow::r::symbols::serialize_arrow_r_metadata, metadata));
    SEXP serialised = PROTECT(Rf_eval(serialise_call, arrow::r::ns::arrow));

    schema = schema->WithMetadata(
        arrow::key_value_metadata({"r"}, {CHAR(STRING_ELT(serialised, 0))}));

    UNPROTECT(2);
  }

  return arrow::Status::OK();
}