yii\validators\UniqueValidator::init PHP Метод

    public function init()
    {
        parent::init();
        if ($this->message !== null) {
            return;
        }
        if (is_array($this->targetAttribute) && count($this->targetAttribute) > 1) {
            // fallback for deprecated `comboNotUnique` property - use it as message if is set
            if ($this->comboNotUnique === null) {
                $this->message = Yii::t('yii', 'The combination {values} of {attributes} has already been taken.');
            } else {
                $this->message = $this->comboNotUnique;
            }
        } else {
            $this->message = Yii::t('yii', '{attribute} "{value}" has already been taken.');
        }
    }