Make get_adjacent_post() work across custom post types

2 comments

1. It appears this question has been asked all over the interweb with no definite answer. So I created my own function from the original get_adjacent_post and customized it for anyone else who needs it.

   The Function

   Drop this in your functions.php

   /*
    * Replacement for get_adjacent_post()
    *
    * This supports only the custom post types you identify and does not
    * look at categories anymore. This allows you to go from one custom post type
    * to another which was not possible with the default get_adjacent_post().
    * Orig: wp-includes/link-template.php 
    * 
    * @param string $direction: Can be either 'prev' or 'next'
    * @param multi $post_types: Can be a string or an array of strings
    */
   function mod_get_adjacent_post($direction = 'prev', $post_types = 'post') {
       global $post, $wpdb;
   
       if(empty($post)) return NULL;
       if(!$post_types) return NULL;
   
       if(is_array($post_types)){
           $txt = '';
           for($i = 0; $i <= count($post_types) - 1; $i++){
               $txt .= "'".$post_types[$i]."'";
               if($i != count($post_types) - 1) $txt .= ', ';
           }
           $post_types = $txt;
       }
   
       $current_post_date = $post->post_date;
   
       $join = '';
       $in_same_cat = FALSE;
       $excluded_categories = '';
       $adjacent = $direction == 'prev' ? 'previous' : 'next';
       $op = $direction == 'prev' ? '<' : '>';
       $order = $direction == 'prev' ? '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 $op %s AND p.post_type IN({$post_types}) AND p.post_status = 'publish'", $current_post_date), $in_same_cat, $excluded_categories );
       $sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );
   
       $query = "SELECT p.* FROM $wpdb->posts AS p $join $where $sort";
       $query_key = 'adjacent_post_' . md5($query);
       $result = wp_cache_get($query_key, 'counts');
       if ( false !== $result )
           return $result;
   
       $result = $wpdb->get_row("SELECT p.* FROM $wpdb->posts AS p $join $where $sort");
       if ( null === $result )
           $result = '';
   
       wp_cache_set($query_key, $result, 'counts');
       return $result;
   }
   

   Usage

   Basic use

   // Custom post types can be array() or string
   $post1 = mod_get_adjacent_post('prev', array('post', 'custom1', 'custom2'));
   $post2 = mod_get_adjacent_post('next', 'custom2');
   

   For creating prev/next links

   <?php
   $prev = mod_get_adjacent_post('prev', array('post', 'custom1', 'custom2'));
   $next = mod_get_adjacent_post('next', array('post', 'custom1', 'custom2'));
   ?>
   
   <?php if($prev) : ?>
       <a href="<?php echo get_permalink($prev->ID)?>">&laquo; Go back in time</a>
   <?php endif; ?>
   
   <?php if($next) : ?>
       <a href="<?php echo get_permalink($next->ID)?>">Next: <?php echo $next->post_title; ?> &raquo;</a>
   <?php endif; ?>
   

   You can still modify the code if you still want to include the variables $in_same_cat and $excluded_categories but if you do then I suggest you use get_adjacent_post instead since that’s what it’s for.

   Log in to Reply

2. The previous answer it’s not longer working. Check this one out

   I came up with a new one, quite more simple:

   • Paste this into your functions.php

   function custom_posttype_get_adjacent_ID($direction = 'next', $type = 'post', $current) {
   
       // Get all posts with this custom post type
       $posts = get_posts('posts_per_page=-1&order=DESC&post_type='.$type);
   
       $postsLength = sizeof($posts)-1;
       $currentIndex = 0;
       $index = 0;
       $result = 0;
   
       // Iterate all posts in order to find the current one
       foreach($posts as $p){
           if($p->ID == $current) $currentIndex = $index;
           $index++;
       }
       if($direction == 'prev') {
           // If it's 'prev' return the previous one unless it's the first one, in this case return the last. 
           $result = !$currentIndex ? $posts[$postsLength]->ID : $posts[$currentIndex - 1]->ID;
       } else {
           // If it's 'next' return the next one unless it's the last one, in this case return the first. 
           $result = $currentIndex == $postsLength ? $posts[0]->ID : $posts[$currentIndex + 1]->ID;
       }
       return $result;
   }
   

   Now wherever you need the next-prev post ID just use the function like this:

   custom_posttype_get_adjacent_ID('prev', 'project', get_the_ID());
   

   Comments:

   • Feel free to replace the get_the_ID()for your current post ID if you feel like it.
   • First parameters expects to be ‘next’ or ‘prev’, it falls back to ‘next’.
   • Second parameter needs to be the custom post type name. You can find it in your register_post_type()function. It falls back to ‘post’.
   • If the last parameter it’s empty it won’t work.

   Examples:

   If you want the next or prev post permalink you can use it like this:

   <?php echo get_permalink(custom_posttype_get_adjacent_ID('prev', 'project', get_the_ID())); ?>
   

   So with the tag it would look something like this:

   <a href="<?php echo get_permalink(custom_posttype_get_adjacent_ID('prev', 'project', get_the_ID())); ?>">Previous Project</a>
   

   Could not test it a lot so if it’s not working in some cases let me know and I’ll try to fix it / improve it.

   Log in to Reply