code: public void add(T object) {
      if (object == null) throw new IllegalArgumentException("null AVObject");
      if (StringUtil.isEmpty(targetClass)) {
        targetClass = object.getClassName();
      }
      if (!StringUtil.isEmpty(targetClass) && !targetClass.equals(object.getClassName())) {
        throw new IllegalArgumentException("Could not add class '" + object.getClassName()
                + "' to this relation,expect class is '" + targetClass + "'");
      }
      parent.addRelation(object, key);
    }
docs: Adds an object to this relation.
  
  @param object The object to add to this relation.