Retrieving post meta with REST API using \register_meta()

Question

WordPress Codex suggests two ways of adding post meta to the response. I'm trying to achieve this with register_meta() since it kinda seems to be the correct way to me.

PROBLEM:

meta field in the response remains empty when I use register_meta() although show_in_rest is set to true.

SOLUTION:

Thanks to Allan Fuller, who pointed out, that the $object_type parameter (the first one in register_meta() function should be post and not the post type. This is also mentioned in the comments section of the new code reference

---

Description before solution

Here how am I doing this:

Step 1: call from functions.php:

\add_action('init', [$this, 'init'], 10);

Step 2: register custom post type in the init function:

\register_post_type(
            CompanyPostType::POST_TYPE_NAME,
            [
                'labels' => [
                    'name' => __('Companies'),
                    'singular_name' => __('Company')
                ],
                'public' => true,
                'show_in_rest' => true,
                'rest_base' => CompanyPostType::REST_BASE,
                'has_archive' => false,
                'rewrite' => false,
                'supports' => [
                    'custom-fields',
                    'revisions'
                ]
            ]
        );

Step 3 - profit: register meta for the custom post type in the init function

$meta = [
     'key' => CompanyPostType::META_NAME,
     'description' => 'Name of the company',
     'type' => 'string'                    
],
\register_meta(CompanyPostType::POST_TYPE_NAME, $meta['key'],
                [
                    'show_in_rest' => true,
                    'single' => true,
                    'type' => $meta['type'],
                    'description' => $meta['description'],
                ]
            );

The meta field is still remains empty in the response. If I however use the second way also suggested in the codex (register_rest_field()) in the same init hook:

\add_action( 'rest_api_init', function () {
            \register_rest_field( CompanyPostType::POST_TYPE_NAME, CompanyPostType::META_NAME, array(
                'get_callback' => function($params ) {
                    $meta = \get_post_meta( $params['id'],  CompanyPostType::META_NAME, true);
                    return (string) $meta;
                },
            ) );
        } );

then the field with its value appears in the response. What am I missing?

Since I'm using a custom post type, it may be worth looking at this part of code as well. As stated in the codex, when registering it I set ‘supports’ => [‘custom-fields’] so that custom fields appear in the rest response. The whole looks like this:

        return \register_post_type(
            self::POST_TYPE_NAME,
            [
                'labels' => [
                    'name' => __('Companies'),
                    'singular_name' => __('Company')
                ],
                'public' => true,
                'show_in_rest' => true,
                'rest_base' => self::REST_BASE,
                'has_archive' => false,
                'rewrite' => false,
                'supports' => [
                    'title',
                    'custom-fields',
                    'revisions'
                ]
            ]
        );

EDIT:

I tried, as suggested by Allan Fuller in his blog post, to put \register_meta() in rest_api_init hook, but it has no effect on meta in the response.

Answer 1

If you want to use register_meta to work, you have to explicitly hook it to rest_api_init
so for your example

add_action( 'rest_api_init', 'register_posts_meta_field' ); 
function register_posts_meta_field() {   
  register_meta('post', $meta['key'],
            [
                'show_in_rest' => true,
                'single' => true,
                'type' => $meta['type'],
                'description' => $meta['description'],
            ]
        );
}

The above is untested but see working plugin version of code here http://badlywired.com/2018/01/getting-post-meta-via-wordpress-rest-api/

The object type needs to be 'post' to the custom post type name

see https://codex.wordpress.org/Function_Reference/register_meta

first argument is object_type - valid object types are predefined by wordpress and CPT is a type of post