Modifing archive query affects show post count function

Question

I'm trying to modify the archive query of archive page with this function hooked to pre_get_posts action hook.

// Load our function when hook is set
add_action( 'pre_get_posts', 'rc_modify_query_limit_posts' );
function rc_modify_query_limit_posts( $query ) {

// Check if on frontend and main query is modified
if( ! is_admin() && $query->is_main_query() && $query->is_archive() ) {

    $query->set('posts_per_page', '9');

}

This is working perfectly, but somehow messes up my function for displaying current posts displayed"

function archive_post_count() {

    global $wp_query;
    $showing = '';

    if ( $wp_query->found_posts > 1 ) {

        $page_number = is_paged() ? $wp_query->query_vars['paged'] : 1;
        $current_max = ( get_option( 'posts_per_page' ) * ( $page_number - 1 ) ) + $wp_query->post_count;
        $current_min = $current_max - $wp_query->post_count + 1;

        $range   = ( $current_min == $current_max ) ? strval( $current_min ) : $current_min . '-' . $current_max;
        $total   = $wp_query->found_posts;

        $showing = 'Shown ' . $range . ' from ' . $total . ' posts ';

    }

    echo $showing;

}

Currently I have 11 posts and on first page everything works OK. But when I hit page 2 "Shown 11-12 from 11 posts" message is shown. Where is my mistake?

Answer 1

pre_get_posts does not explicitely changes the value in the db of posts_per_page, this value stays constant to the value set in the back end under the readings settings. pre_get_posts only changes this value before the SQL query is build in WP_Query right before the main query runs.

If you need to get the exact ( more specific ) amount of posts per page set for a specific page when using pre_get_posts, access the query vars which will hold the new value set by pre_get_posts. (NOTE: query_posts breaks the main query object, so if you use query_posts somewhere on the page, this will also fail as you will get an incorrect value. This is ONE BIG REASON why you should never ever use query_posts Here is an answer that explains this in detail.)

Instead of using

get_option( 'posts_per_page' );

use

$wp_query->query_vars['posts_per_page'];