public function changed($field = null)
    {
        if ($this->owner->isNewRecord)
            return false;
        if ($field)
            return $this->getDbAttribute($field) != $this->owner->getAttribute($field);
        foreach ($this->owner->getAttributes() as $k => $v)
            if ($this->getDbAttribute($k) != $v)
                return true;
        return false;
    }