For previous and next post links to work, you need to hook on to the get_*_post_*
filters and modify the query to actually sort the posts using your custom field. I hope the code below will work for you.
For Previous post link
add_filter('get_previous_post_join', 'wpse96670_get_previous_post_join_custom_sort');
add_filter('get_previous_post_where', 'wpse96670_get_previous_post_where_custom_sort');
add_filter('get_previous_post_sort', 'wpse96670_get_previous_post_sort_custom_sort');
function wpse96670_get_previous_post_join_custom_sort($join) {
global $wpdb;
return "INNER JOIN {$wpdb->prefix}postmeta ON (p.ID = {$wpdb->prefix}postmeta.post_id)";
}
function wpse96670_get_previous_post_where_custom_sort($where) {
global $wpdb, $post;
$meta_value = get_post_meta($post->ID, 'aaaConsultant_dateofletter', true);
return "WHERE 1=1 AND p.post_type="letters_of_ref" AND p.post_status="publish" AND {$wpdb->prefix}postmeta.meta_key = 'aaaConsultant_dateofletter' AND {$wpdb->prefix}postmeta.meta_value < '{$meta_value}'";
}
function wpse96670_get_previous_post_sort_custom_sort($sort) {
global $wpdb;
return "ORDER BY {$wpdb->prefix}postmeta.meta_value DESC LIMIT 1";
}
For next post link
add_filter('get_next_post_join', 'wpse96670_get_next_post_join_custom_sort');
add_filter('get_next_post_where', 'wpse96670_get_next_post_where_custom_sort');
add_filter('get_next_post_sort', 'wpse96670_get_next_post_sort_custom_sort');
function wpse96670_get_next_post_join_custom_sort($join) {
global $wpdb;
return "INNER JOIN {$wpdb->prefix}postmeta ON (p.ID = {$wpdb->prefix}postmeta.post_id)";
}
function wpse96670_get_next_post_where_custom_sort($where) {
global $wpdb, $post;
$meta_value = get_post_meta($post->ID, 'aaaConsultant_dateofletter', true);
return "WHERE 1=1 AND p.post_type="letters_of_ref" AND p.post_status="publish" AND {$wpdb->prefix}postmeta.meta_key = 'aaaConsultant_dateofletter' AND {$wpdb->prefix}postmeta.meta_value > '{$meta_value}'";
}
function wpse96670_get_next_post_sort_custom_sort($sort) {
global $wpdb;
return "ORDER BY {$wpdb->prefix}postmeta.meta_value ASC LIMIT 1";
}
Then you need some code in the template:
<?php
$oNext = get_adjacent_post(false, '', true);
$oPrev = get_adjacent_post(false, '', false);
if ($oPrev || $oNext) {
echo '<div id="lor_nav">' . "\n";
}
if ($oPrev) {
echo '<div class="prevlink"><a href="' . get_permalink($oPrev->ID) . '"> « ' . $oPrev->post_title . '</a></div>' . "\n";
}
if ($oNext) {
echo '<div class="nextlink"><a href="' . get_permalink($oNext->ID) . '">' . $oNext->post_title . ' »</a></div>' . "\n";
}
if ($oPrev || $oNext) {
echo '</div><div class="clear"></div>' . "\n";
}
?>
Let me know if you need any clarification.