function make_columns_sortable( $columns ) {

        // For each sortable column.
        foreach ( $this->sortable as $column => $values ) {

            // Make an array to merge into wordpress sortable columns.
            $sortable_columns[ $column ] = $values[0];
        }

        // Merge sortable columns array into wordpress sortable columns.
        $columns = array_merge( $sortable_columns, $columns );

        return $columns;
    }