public function getSourceRules()
    {
        if (!is_string($this->sourceAttribute) || empty($this->sourceAttribute)) {
            return [];
        }
        if (empty($this->_sourceRules)) {
            $this->_sourceRules = [
                [[$this->sourceAttribute], 'required'],
                [[$this->sourceAttribute], 'string'],
            ];
        }
        return $this->_sourceRules;
    }