Get Previous & Next posts by Post ID

Question

How can I go about getting the previous and next posts from outside the loop? Everything I try to do aside from the custom SELECT query (using get_posts or WP_Query) breaks stuff further down the page.

I currently have a recursive function which will hopefully find the first "previous" post which satisfies a certain condition. I grab the first value to test from get_previous_post() but don't know how to get the previous_previous_post().

add_action( 'wp_ajax_nopriv_myajax-submit', 'ajax_project_load' );
add_action( 'wp_ajax_myajax-submit', 'ajax_project_load' );

function ajax_project_load() {
    $the_slug = $_POST['slug'];
    $args=array(
      'name' => $the_slug,
      'post_type' => 'projects',
      'post_status' => 'publish',
      'showposts' => 1,
      'ignore_sticky_posts' => 1
    );
    $my_posts = get_posts($args);
    if( $my_posts ) :

        global $post;
        $post = $my_posts[0]; 
        $postCapabilityFilter = $_POST['capFilter']=='undefined' ? $_POST['capFilter'] : substr($_POST['capFilter'], 1);

        $response = json_encode( "Success" );
        header( "Content-Type: application/json" );     


$next_post = get_previous_post();
function filtered_next_post($next_post){
    if($next_post){
        $next_slug = $next_post->post_name;
        $next_ID = $next_post->ID;
        global $wpdb;
        global $postCapabilityFilter;
        $querystr = "
        SELECT $wpdb->postmeta.*
        FROM $wpdb->posts, $wpdb->postmeta
        WHERE $wpdb->posts.post_type = 'projects' 
        AND $wpdb->posts.post_status = 'publish' 
        AND $wpdb->posts.ID = $next_ID
        AND $wpdb->posts.ID = $wpdb->postmeta.post_id
        AND $wpdb->postmeta.meta_key = '_slideshow_content'
        ORDER BY $wpdb->posts.post_name DESC
        ";
        $nextProjQuery = $wpdb->get_results($querystr, OBJECT);
        foreach($nextProjQuery as $nextProj):
            $nextProjMetaArr = unserialize($nextProj->meta_value);
            foreach ($nextProjMetaArr['slides'] as $npSlideArr){
                echo 'and..';
                if ( !in_array( $postCapabilityFilter, $npSlideArr['slideCap'] ) ){
                    echo 'not in it..';
                    //$next_post = get_previous_previous_post();
                    //filtered_next_post($next_post);
                }
            }
        endforeach;                        
        return $next_slug;
    }
}
$next_slug = filtered_next_post($next_post);

Answer 1

Take a look at get_previous_post() and get_next_post() and you'll see they both use the get_adjacent_post() to find the previous or next post.

Let's say you want to fetch the ID of the immediately previous post based on the current post's ID. This is what you'd do:

function get_previous_post_id( $post_id ) {
    // Get a global post reference since get_adjacent_post() references it
    global $post;

    // Store the existing post object for later so we don't lose it
    $oldGlobal = $post;

    // Get the post object for the specified post and place it in the global variable
    $post = get_post( $post_id );

    // Get the post object for the previous post
    $previous_post = get_previous_post();

    // Reset our global object
    $post = $oldGlobal;

    if ( '' == $previous_post ) {
        return 0;
    }

    return $previous_post->ID;
}

You can do a similar thing to fetch the next post's ID ... and you can do this recursively if you need to get the previous previous post:

$two_posts_ago = get_previous_post_id( get_previous_post_id( $post->ID ) );

TL;DR

Essentially, both get_previous_post() and get_next_post() reference a global $post object to do their selection. You need to set this object up before calling either function or they won't know what post to use as a reference for next/previous.

The wrapper function above just sets up the global $post for you based on a passed-in ID. You could have it return the entire object for the previous post rather than the ID, it's entirely up to you.

Answer 2

Here, you can get adjacent post for specific post type with custom sql query & with filter get_{$adjacent}_post_where where default adjacent is previous. Also result depends on $current_post_date & comparison operator $op.

function bm_get_adjacent_post( $post_id, $author_id, $previous = 1 ) {
global $wpdb;

if ( ( ! $post = get_post( $post_id ) )  )
    return null;

$current_post_date = $post->post_date;

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

$where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare( "WHERE  p.post_date $op %s AND p.post_type = %s AND p.post_status = 'publish' AND p.post_author = %d", $current_post_date, 'projects' , $author_id ), '', '' );
$sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );

$query = "SELECT p.ID FROM $wpdb->posts AS p $where $sort";
//echo $query;
$result = $wpdb->get_var( $query );
if ( null === $result )
    $result = '';
if ( $result )
    $result = get_post( $result );

return $result;
}

Answer 3

I love this answer found at: https://stackoverflow.com/a/33688032/2062851

function get_previous_post_id( $post_id ) {
    // Get a global post reference since get_adjacent_post() references it
    global $post;
    // Store the existing post object for later so we don't lose it
    $oldGlobal = $post;
    // Get the post object for the specified post and place it in the global variable
    $post = get_post( $post_id );
    // Get the post object for the previous post
    $previous_post = get_previous_post();
    // Reset our global object
    $post = $oldGlobal;
    if ( '' == $previous_post ) 
        return 0;
    return $previous_post->ID; 
} 

function get_next_post_id( $post_id ) {
    // Get a global post reference since get_adjacent_post() references it
    global $post;
    // Store the existing post object for later so we don't lose it
    $oldGlobal = $post;
    // Get the post object for the specified post and place it in the global variable
    $post = get_post( $post_id );
    // Get the post object for the next post
    $next_post = get_next_post();
    // Reset our global object
    $post = $oldGlobal;
    if ( '' == $next_post ) 
        return 0;
    return $next_post->ID; 
} 

then all you would do is call the function like:

echo get_previous_post_id( $current_id );
echo get_next_post_id( $current_id );