pre_get_posts shows posts in random order sometimes

Question

I'm using the following filter in order to increase the amount of posts shown on category templates from 10 to 30 (I want to keep 10 for the rest of the site)

// Modify number of results shown
function modify_query_amount_shown($query){
    if ($query->is_category) {
        $query->set('posts_per_page', 30);
        $query->set('orderby', 'menu_order');
    }
    return $query;
}
add_filter('pre_get_posts', 'modify_query_amount_shown');

However, when I refresh my page several times, I notice the order of my posts changes each time. I can confirm that it has something to do with this code, because as soon as I comment it out, it returns to normal.

Answer 1

pre_get_posts is not a filter hook, it is an action. Also you should not set orderby to menu_order for posts as they are not hierarchical. Set orderby to date instead.

function modify_query_amount_shown( $query ) {
    if ( $query->is_category ) {
        $query->set( 'posts_per_page', 30 );
        $query->set( 'orderby', 'date' );
    }
}
add_action( 'pre_get_posts', 'modify_query_amount_shown' );

Now the order of posts displayed will be consistent, regardless of the number of refreshes.