Wordpress Custom Ordering Post Link Functions - wordpress

Question

I'm using a custom post type to hold products, each product holds a name via a custom field. When I display the product via its singular template it would be nice to have a next & previous link to go through those products, but it should go through them in order of their name. Currently that’s not possible as they would be ordered by their publishing dates.
So I added the following code in functions.php, but it's not working, I don't know if this has to be made in a different way:
if(is_singular('products')) {
add_filter('get_previous_post_join', 'rt_post_join');
add_filter('get_next_post_join', 'rt_post_join');
add_filter('get_previous_post_where', 'rt_prev_post_where');
add_filter('get_next_post_where', 'rt_next_post_where');
add_filter('get_previous_post_sort', 'rt_prev_post_sort');
add_filter('get_next_post_sort', 'rt_next_post_sort');
}
function rt_post_join($join, $isc, $ec) {
global $wpdb;
$join = " INNER JOIN $wpdb->postmeta AS pm ON pm.post_id = p.ID";
return $join;
}
function rt_prev_post_where($w) {
global $wpdb, $post;
$prd = get_post_meta($post->ID, 'data_product_name_product', true);
$w = $wpdb->prepare(" WHERE pm.meta_key = 'data_product_name_product' AND pm.meta_value < '$prd' AND p.post_type = 'products' AND p.post_status = 'publish'");
return $w;
}
function rt_next_post_where($w) {
global $wpdb, $post;
$prd = get_post_meta($post->ID, 'data_product_name_product', true);
$w = $wpdb->prepare(" WHERE pm.meta_key = 'data_product_name_product' AND pm.meta_value > '$prd' AND p.post_type = 'products' AND p.post_status = 'publish'");
return $w;
}
function rt_prev_post_sort($o) {
$o = "ORDER BY pm.meta_value DESC LIMIT 1";
return $o;
}
function rt_next_post_sort($o) {
$o = "ORDER BY pm.meta_value ASC LIMIT 1";
return $o;
}
And in my single page of products (single-products.php) I added the following code to display the pagination links:
<?php next_post_link('%link', 'Next product'); ?>
<?php previous_post_link('%link','Previous product'); ?>
Thanks.

Answer 1

What you actually need is a custom Query. You can try this (not tested tho):
Place this in your functions.php:
function getProductsQuery() {
return new WP_Query(array(
'posts_per_page' => 10,
'post_type' => 'product',
'orderby' => 'title',
'order' => 'ASC'
));
}
Then, replace your loop with the following:
<?php $products = getProductsQuery(); ?>
<?php while($products->have_posts()): $products->the_post(); ?>
<h2><?php the_title(); ?></h2>
(...)
<?php endwhile; ?>

Answer 2

Finally I solved it!
I just used the plugin Next/Previous Post Link Plus for WordPress and added the following code in my single page of products (single-products.php):
<?php next_post_link_plus( array(
'order_by' => 'custom',
'meta_key' => 'data_product_name_product',
'link' => 'Next product',
'format' => '%link'
)); ?>
<?php previous_post_link_plus( array(
'order_by' => 'custom',
'meta_key' => 'data_product_name_product',
'link' => 'Previous product',
'format' => '%link'
)); ?>