Allow wordpress posts to have hierarchy - figures out slugs

custom-post-hierarchy.php

<?php

add_action('registered_post_type', 'make_posts_hierarchical', 99, 2);

/**
 * Ensure posts post type is hierarchal and allows page attributes
 *
 * Initial Setup - Runs after each post type is registered
 */
function make_posts_hierarchical($post_type, $pto)
{

  // Return, if not post type posts
  if ($post_type != 'post') return;

  // access $wp_post_types global variable
  global $wp_post_types;

  // Set post type "post" to be hierarchical
  $wp_post_types['post']->hierarchical = 1;

  // Add page attributes to post backend
  // This adds the box to set up parent and menu order on edit posts.
  add_post_type_support('post', 'page-attributes');
}

/**
 *
 * Edit View of Permalink
 *
 * This affects editing permalinks, and $permalink is an array [template, replacement]
 * where replacement is the post_name and template has %postname% in it.
 *
 **/
add_filter('get_sample_permalink', function ($permalink, $post_id, $title, $name, $post) {
  if ($post->post_type != 'post' || !$post->post_parent) {
    return $permalink;
  }

  // Deconstruct the permalink parts
  $template_permalink = current($permalink);
  $replacement_permalink = next($permalink);

  // Find string
  $postname_string = '%postname%';
  $altered_template_with_parent_slug = get_path_from_post_id($post->ID, $postname_string);
  $new_template = str_replace("/$postname_string/", "/$altered_template_with_parent_slug/", $template_permalink);

  $new_permalink = [$new_template, $replacement_permalink];

  return $new_permalink;
}, 99, 5);

/**
 * Alter the link to the post
 *
 * This affects get_permalink, the_permalink etc.
 * This will be the target of the edit permalink link too.
 *
 * Note: only fires on "post" post types.
 */
add_filter('post_link', function ($post_link, $post, $leavename) {

  if ($post->post_type != 'post' || !$post->post_parent || $post->post_status == 'draft') {
    return $post_link;
  }

  // this filter can be applied when we want the templating for slug and also when we don't.
  // so check if the templating is there and continue to support it if so.
  $post_slug = stristr($post_link, '%postname%') ? '%postname%' : $post->post_name;
  $path = get_path_from_post_id($post->ID, $post_slug);

  return home_url($path);
}, 99, 3);

/**
 * Before getting posts
 *
 * Has to do with routing... adjusts the main query settings
 *
 */
add_action('pre_get_posts', function ($query) {
  global $wp_query;

  $original_query = $query;
  $uri_with_query_string = $_SERVER['REQUEST_URI'];
  $query_string = $_SERVER['QUERY_STRING'];
  $uri = str_replace('?' . $query_string, '', $uri_with_query_string);

  // Do not do this post check all the time
  if ($uri != '/' && $query->is_main_query() && !is_admin()) {

    $post = get_post_from_uri($uri);

    if (!$post) {
      return $original_query;
    }

    // pretty high confidence that we need to override the query.
    $query->query_vars['post_type'] = ['post'];
    $query->is_home    = false;
    $query->is_attachment = false;
    $query->is_page    = true;
    $query->is_single  = true;
    $query->is_404     = false;
    $query->queried_object_id = $post->ID;
    $query->set('page_id', $post->ID);

    $wp_query = $query;

    return $query;
  }

  return $wp_query;
}, 0);

add_filter('preview_post_link', 'preview_redirect_fix');

function preview_redirect_fix($url)
{
  global $post;

  if ($post->post_status == 'draft') {

    $pieces = (object) parse_url($url);
    $url = implode('', [
      $pieces->scheme,
      '://',
      $pieces->host . '/index.php?',
      $pieces->query
    ]);
  }

  return $url;
}

function get_path_from_post_id($pid, $current_slug = '')
{
  $pid_original = $pid;
  $slugs = [];
  while (!empty($pid)) {
    // Load the post for pid
    $p = get_post($pid);
    // Allow the original post to have a different name (useful for templating %postname% in permalink preview)
    $slugs[] = $pid == $pid_original && !empty($current_slug)
      ? $current_slug : $p->post_name;
    // Setup parent post id as new pid
    $pid = $p->post_parent;
  }

  return implode('/', array_reverse($slugs));
}

function get_post_from_uri($uri)
{
  global $wpdb;

  $basename = basename($uri);
  $depth = count(explode('/', trim($uri, '/')));

  // This inner query says "go get all posts - (ID, SLUG, PARENT_ID) - where slug = (last part of url)"
  $baseQuery = sprintf("select id, post_name as p1_slug, post_parent as p1_parent
      from $wpdb->posts where post_type = '%s' and post_name = '%s'", 'post', $wpdb->_real_escape($basename));

  // We will use concat to make slugs out of the results!
  // We will array_reverse the concats and implode with '/' to make the slug.
  $concats = [];
  $concats[] = "IFNULL(p1_slug, '')";

  // initialize our SQL string with the base query
  $sql = $baseQuery;

  // We will do 1 more depth level than we need to confirm the slug would not lazy match
  // This for loop builds inside out.
  for ($c = 1; $c < $depth + 2; $c++) {
    $d = $c;
    $p = $c + 1;

    $pre = "select d${d}.*, p${p}.post_name as p${p}_slug, p${p}.post_parent as p${p}_parent from (";
    $suf = ") as d${d} left join $wpdb->posts p${p} on p${p}.id = d${d}.p${c}_parent";

    $sql = $pre . $sql . $suf;

    $concats[] = sprintf("IFNULL(p${p}_slug,'')");
  }

  $trimmedUri = trim($uri, '/');
  $concatSql = implode(", '/',", array_reverse($concats));
  $finalSql = "select * from (select TRIM(BOTH '/' FROM
    concat($concatSql)) as slug, id from ($sql) as d${c}) as all_slugs
    where slug = '$trimmedUri';";

  $result = $wpdb->get_results($finalSql);
  if (empty($result) || !($post = current($result))) {
    return false;
  }

  return get_post($post->id);
}

Hello,
I wanted a plugin that will enable me to set a post as the parent of another post. Would this tweak be helpful?
How about making this tweak into a plugin since it would be difficult to use it the way it is?
I would appreciate your response. Thanks

@Githubeys you only need to get a plugin that handles code snippets. And than paste it as php snippet in there.
@amurrell for some reason your previous version works but the above does not seem to do anything at all.
Can you help me troubleshoot please?

The previous version:

add_action('registered_post_type', 'make_posts_hierarchical', 10, 2 );

// Runs after each post type is registered
function make_posts_hierarchical($post_type, $pto){

    // Return, if not post type posts
    if ($post_type != 'post') return;

    // access $wp_post_types global variable
    global $wp_post_types;

    // Set post type "post" to be hierarchical
    $wp_post_types['post']->hierarchical = 1;

    // Add page attributes to post backend
    // This adds the box to set up parent and menu order on edit posts.
    add_post_type_support( 'post', 'page-attributes' );

}

/**
 * Get parent post slug
 * 
 * Helpful function to get the post name of a posts parent
 */
function get_parent_post_slug($post) {
  if (!is_object($post) || !$post->post_parent) {
    return false;
  }

  return get_post($post->post_parent)->post_name;
}

/**
 * 
 * Edit View of Permalink
 * 
 * This affects editing permalinks, and $permalink is an array [template, replacement]
 * where replacement is the post_name and template has %postname% in it.
 * 
 **/
add_filter('get_sample_permalink', function($permalink, $post_id, $title, $name, $post) {
  if ($post->post_type != 'post' || !$post->post_parent) {
    return $permalink;
  }

  // Deconstruct the permalink parts
  $template_permalink = current($permalink);
  $replacement_permalink = next($permalink);

  // Find string
  $postname_string = '/%postname%/';

  // Get parent post
  $parent_slug = get_parent_post_slug($post);

  $altered_template_with_parent_slug = '/' . $parent_slug . $postname_string;
  $new_template = str_replace($postname_string, $altered_template_with_parent_slug, $template_permalink);

  $new_permalink = [$new_template, $replacement_permalink];

  return $new_permalink;
}, 99, 5);

/**
 * Alter the link to the post
 * 
 * This affects get_permalink, the_permalink etc. 
 * This will be the target of the edit permalink link too.
 * 
 * Note: only fires on "post" post types.
 */
add_filter('post_link', function($post_link, $post, $leavename){

  if ($post->post_type != 'post' || !$post->post_parent) {
    return $post_link;
  }
  
  $parent_slug = get_parent_post_slug($post);
  $new_post_link = str_replace($post->post_name, $parent_slug . '/' . $post->post_name, $post_link);

  return $new_post_link;
}, 99, 3);

/**
 * Before getting posts
 * 
 * Has to do with routing... adjusts the main query settings
 * 
 */
add_action('pre_get_posts', function($query){
  global $wpdb, $wp_query;

  $original_query = $query;
  $uri = $_SERVER['REQUEST_URI'];

  // Do not do this post check all the time
  if ( $query->is_main_query() && !is_admin()) {

    // get the post_name
    $basename = basename($uri);
    // find out if we have a post that matches this post_name
    $test_query = sprintf("select * from $wpdb->posts where post_type = '%s' and post_name = '%s';", 'post', $basename);
    $result = $wpdb->get_results($test_query);

    // if no match, return default query, or if there's no parent post, this is not necessary
    if (!($post = current($result)) || !$post->post_parent) {
      return $original_query;
    }

    // get the parent slug
    $parent_slug = get_parent_post_slug($post);
    // concat the parent slug with the post_name to get most of the url
    $hierarchal_slug = $parent_slug . '/' . $post->post_name;

    // if the concat of parent-slug/post-name is not in the uri, this is not the right post.
    if (!stristr($uri, $hierarchal_slug)) {
      return $original_query;
    }

    // pretty high confidence that we need to override the query.
    $query->query_vars['post_type'] = ['post'];
    $query->is_home     = false;
    $query->is_attachment = false;
    $query->is_page     = true;  
    $query->is_single   = true; 
    $query->queried_object_id = $post->ID;  
    $query->set('page_id', $post->ID);

    return $query;
  }


}, 1);