pre_get_posts works in post type archive but not in single post

Question

I have a custom post type "Scholars" that has an archive. I want to sort by a custom meta field called "year", I'm doing this with pre_get_posts:

add_action('pre_get_posts', 'my_queries');
function my_queries($query)
{
  if (!$query->is_main_query()) return;

  if ($query->query['post_type'] == 'scholars') {
    $query->set('posts_per_page', -1);
    $query->set('meta_key', 'year');
    $query->set('orderby', 'meta_value');
  }
}

This works fine in the archive page (the main listing), but the next_post_link and previous_post_link are not in sync in the single post. They are still sorted by date and not by custom "year".

I have a custom query on the single page using WP_Query to show related posts, but I tried removing it and still does not work.

This is the archive page:

<div class="large-12 columns">
  <?php
    if (have_posts()) {
      while (have_posts()) {
        the_post();
        // this just loads an <article>
        get_template_part('content', 'scholars');
      }
    } else {
      echo '<h2>Nothing found.</h2>';
    }
  ?>
</div>

And this is the single page:

<?php the_post() ?>

<div class="large-12 columns">
  <nav class="scholars-navigation">
    <span class="prev"><?php previous_post_link() ?></span>
    <h2><?php the_title() ?></h2>
    <span class="next"><?php next_post_link() ?></span>
  </nav>
</div>

<div class="large-3 columns scholar-info">
  <img src="<?php the_field('photo') ?>" alt="" />
  <h3><?php the_field('year') ?> Scholar</h3>
</div>

Any ideas why it won't work?

Edit: After looking at other related answers seems like this is not possible with the built-in function out of the box, found this plugin but I'm open to any solution.

Edit: Pfff, it doesn't even work with that plugin, don't know what the issue is:

next_post_link_plus('meta_key=year&order_by=numeric')

Answer 1

The next_post_link() and previous_post_link() functions do not use the same query that 'pre_get_posts' affects. They use their own custom queries. You have to use other filters to affect them.

Utimately, the get_adjacent_post() function does the bulk of the work for the next_post_link() and previous_post_link() functions.

Here is part of that function:

$adjacent = $previous ? 'previous' : 'next';
$op = $previous ? '<' : '>';
$order = $previous ? 'DESC' : 'ASC';

$join  = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_cat, $excluded_categories );
$where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare("WHERE p.post_date $op %s AND p.post_type = %s AND p.post_status = 'publish' $posts_in_ex_cats_sql", $current_post_date, $post->post_type), $in_same_cat, $excluded_categories );
$sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );

To change the sort order for the next post use the 'get_next_post_sort' filter and use the 'get_previous_post_sort' filter to change the sort for the previous link.

You can see from this code snippet that the value in $order in the original query depends on which filter you are writing code for.

Sorry, I don't know SQL well enough to write that part of the query.