add_action('pre_get_posts', 'td_modify_main_query_for_category_page');
function td_modify_main_query_for_category_page2($query) {


    //checking for category page and main query
    if(!is_admin() and is_category() and $query->is_main_query()and (is_front_page() and is_home())) {

        // get the category object - with or without permalinks
        if (empty($query->query_vars['cat'])) {
            td_global::$current_category_obj = get_category_by_path(get_query_var('category_name'), false);  // when we have permalinks, we have to get the category object like this.
        } else {
            td_global::$current_category_obj = get_category($query->query_vars['cat']);
        }


        // we are on a category page with an ID that doesn't exists - wp will show a 404 and we do nothing
        if (is_null(td_global::$current_category_obj)) {
            return;
        }


        //get the number of page where on
        $paged = get_query_var('paged');

        //get the `filter_by` URL($_GET) variable
        $filter_by = get_query_var('filter_by');

        //get the limit of posts on the category page
        $limit = get_option('posts_per_page');


        //echo $filter_by;
        switch ($filter_by) {
            case 'featured':
                //get the category object
                $query->set('category_name',  td_global::$current_category_obj->slug);
                $query->set('cat', get_cat_ID(TD_FEATURED_CAT)); //add the fetured cat
                break;

            case 'popular':
                $query->set('meta_key', td_page_views::$post_view_counter_key);
                $query->set('orderby', 'meta_value_num');
                $query->set('order', 'DESC');
                break;

            case 'popular7':
                $query->set('meta_key', td_page_views::$post_view_counter_7_day_total);
                $query->set('orderby', 'meta_value_num');
                $query->set('order', 'DESC');
                break;

            case 'review_high':
                $query->set('meta_key', td_review::$td_review_key);
                $query->set('orderby', 'meta_value_num');
                $query->set('order', 'DESC');
                break;

            case 'random_posts':
                $query->set('orderby', 'rand');
                break;
        }//end switch


        // how many posts are we showing in the big grid for this category
        $offset = 5;


        // offset + custom pagination - if we have offset, WordPress overwrites the pagination and works with offset + limit
        if(empty($query->is_feed)) {
            if ( ! empty( $offset ) and $paged > 1 ) {
                $query->set( 'offset', $offset + ( ( $paged - 1 ) * $limit ) );
            } else {
                $query->set( 'offset', $offset );
            }
        }
        //print_r($query);
    }//end if main query
}