code: public Attribute getIdAttribute() {
      Attribute idAttr = getOwnIdAttribute();
      if (idAttr == null) {
        EntityType extend = getExtends();
        if (extend != null) {
          idAttr = extend.getIdAttribute();
        }
      }
      return idAttr;
    }
docs: Attribute that is used as unique Id. Id attribute should always be provided.
  
  @return id attribute