public function head()
    {
        $head = '';
        $head .= '<thead>';
        $head .= '<tr class="resource-table-headings">';
        foreach ($this->_getColumns() as $column) {
            $head .= '<th class="'.($column->sortActive() ? 'warning' : '').'">';
            $head .= $column->content();
            $head .= '</th>';
        }
        $head .= '</tr>';

        if ($this->_config['filter']) {
            // Render row with search inputs
            $head .= '<tr class="resource-table-filter">';

            foreach ($this->_getColumns() as $column) {
                // Render each column

                if (!$column->searchable()) {
                    // Column is not searchable - do not print form elements
                    $head .= '<td></td>';
                    continue;
                }

                $head .= '<td>'.$column->searchableContent().'</td>';
            }

            $head .= '</tr>';
        }

        $head .= '</thead>';
        return $head;
    }