private void deriveImplicitFields(DocumentSummary summary, Map<String, SummaryClassField> fields) {
        if (summary.name().equals("default")) {
            addField(SummaryClass.DOCUMENT_ID_FIELD, DataType.STRING, SummaryElementsSelector.selectAll(), SummaryTransform.DOCUMENT_ID, "", fields);
        }
    }
MUST be called after all other fields are added
config-model/src/main/java/com/yahoo/schema/derived/SummaryClass.java