REST API: Expose old slugs

[crosescu]

Expose the old slugs for all posts including custom post types in the REST API.

For example, when building a React-powered app using the WordPress REST API the old slugs are required so that users can be automatically redirected if they clicked on an old link.

To expose the old slugs I created a small plugin (see below):

<?php
add_action( 'rest_api_init', 'expose_old_slugs_in_api' );
 
function expose_old_slugs_in_api() {
  $post_types = get_post_types( array( 'show_in_rest' => true, 'show_in_nav_menus' => true ) ) ;

  $args = array(
    'get_callback' => 'get_old_slugs_for_api', 
    'schema' => null
  );
  
  foreach ($post_types as $type) {    
    register_rest_field( $type, 'old_slugs', $args );
  }  
}
 
function get_old_slugs_for_api( $object ) {
  //get the id of the post object array
  $post_id = $object['id'];

  //return the post meta
  return get_post_meta( $post_id, '_wp_old_slug' );
}

[rmccue]

I see no reason not to add this, so long as it's only available in the edit context.

Add old_slug to list of fields returned with post details.

Trac ticket: https://core.trac.wordpress.org/ticket/41616

[LeonidasMilossis]

I see the PR considers and edits unit tests, should we remove the needs-unit-tests tag?

Add old_slug to REST API fields.

This is a review of the changes made in #1164. The code was made from those changes then resolving conflicts and incorporating feedback from the previous patch.

Trac ticket: core.trac.wordpress.org/ticket/41616

[xipasduarte]

Resolved conflict issues and incorporated the following feedback:

• Available only in context edit (@rmccue)
• Allow null value ([@KiOui](​https://github.com/KiOui))

[xipasduarte]

Revised code with feedback and test changes

[xipasduarte]

Add missing changes to generated fixtures