Make get_adjacent_post() work across custom post types

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.

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.