WP_Query orderby random do not repeat infinite scroll - one loop

Question

I have found a lot of documentation on solving this issue with multiple loops, I'm finding it hard to see a solution for one custom loop.

Problem: I'm using infinite scroll to load in the next page of posts to the homepage. The posts need to be shown in a random order so I need to store which posts have been shown and then exclude them the second time the query is hit.

<?php $my_query = new WP_Query(array('post__not_in'=> $do_not_duplicate, 
                        'orderby' => 'rand',
                        'category_name' => 'projects', 
                        'posts_per_page' => 20, 
                        'paged' => $paged));
while ($my_query->have_posts()) : $my_query->the_post();
$do_not_duplicate = $post->ID; ?>

<?php get_template_part( 'entry' ); ?>
<?php endwhile; ?>

I have built this from http://codex.wordpress.org/The_Loop#Multiple_Loops_in_Action - however the approach there is to use $do_not_duplicate in the second loop.

Perhaps I need to shuffle this around a bit, store the posts shown in another way, a cookie? Any guidance appreciated.

Answer 1

I use this solution in my functions.php

 /*------ order ------*/
session_start();

add_filter('posts_orderby', 'edit_posts_orderby');

function edit_posts_orderby($orderby_statement) {

    $seed = $_SESSION["sem"];

        if (empty($seed)) {
          $seed = rand();
          $_SESSION["sem"] = $seed;
        }

        $orderby_statement = 'RAND('.$seed.')';
        return $orderby_statement;  

}

And additionaly, in the page where you show the posts, in the top of the file, i reset the session variable to can start ordering again random in each reload at this way:

$semilla = rand();

$_SESSION["sem"] = $semilla;

Works for me very well.