Nette\Database\Structure::analyzePrimaryKey PHP Method

    protected function analyzePrimaryKey(array $columns)
    {
        $primary = [];
        foreach ($columns as $column) {
            if ($column['primary']) {
                $primary[] = $column['name'];
            }
        }
        if (count($primary) === 0) {
            return NULL;
        } elseif (count($primary) === 1) {
            return reset($primary);
        } else {
            return $primary;
        }
    }