public function addUnique($column, $options = []) {
        $symbol = '';
        $index = $column;

        if (is_array($options)) {
            if (isset($options['constraint'])) {
                $symbol = $options['constraint'];
            }

            if (isset($options['index'])) {
                $index = $options['index'];
            }
        } else if (is_string($options)) {
            $index = $options;
        }

        if (empty($this->_uniqueKeys[$index])) {
            $this->_uniqueKeys[$index] = [
                'index' => $index,
                'constraint' => $symbol,
                'columns' => [$column]
            ];
        } else {
            $this->_uniqueKeys[$index]['columns'][] = $column;
        }
    }