I have a custom meta box for saving values from two input fields into a single post meta field.

The data is saved serialized in the DB:

get_post_meta( $post->ID, '_custom_key', true )

will return:

a:2:{s:5:"email";s:10:"[email protected]";s:6:"number";i:15;}

Now, with Gutenberg, I want to move that whole custom meta box into a separate sidebar plugin. So in order to do this, first, I had to add the __back_compat_meta_box argument when adding the meta box, so it won't appear in Sidebar -> Settings -> Document anymore:

        [ $this, 'renderOptionsBox' ],
        [ 'post', 'page', 'attachement' ], // the meta field is registered for multiple post types 
        [ '__back_compat_meta_box' => true ]

For the meta field (_custom_key) to work with the block editor, it has to be registered using the register_meta function:

register_meta( string $object_type, string $meta_key, array $args, string|array $deprecated = null )

My questions are:

  1. [updated] How can I register the custom meta field for all of the post types in this list [ 'post', 'page', 'attachement' ]?
  2. How can I still follow the same serialized way of saving the data in a single field if the types ($args->type) supported when registering a meta are only 'string', 'boolean', 'integer', and 'number'?

Right now if I just register the _custom_key meta using:

    register_meta( 'post', '_custom_key', [
        'show_in_rest' => true,
        'single' => true,
        'type' => 'string',
        'auth_callback' => function () {
            return current_user_can( 'edit_posts' );

I will get a Notice: Array to string conversion in .../wp-includes/rest-api/fields/class-wp-rest-meta-fields.php because already saved data from the DB is serialized.

and typing in the console: wp.data.select( 'core/editor' ).getCurrentPost().meta; will return a string: {_custom_key: "Array"}.

  • 1
    I don't know the answer to 2 (as far as I can tell, register_meta() does not support array values), but in regards to 1, object_type is not the same as post_type. The object type refers to whether it's for a post type, user or term. If you want to register it for specific post types, set object_type to post and object_subtype (in the args array) to the post type. Mar 6, 2019 at 13:39
  • object_subtype still accepts only a string. Does that mean that I have to register the meta for each post I want?
    – Vlad
    Mar 6, 2019 at 14:50
  • Seems that way. Mar 6, 2019 at 14:59

2 Answers 2


Here's an answer to question 1 that uses register_post_meta. It's also the start of an answer to question 2 by use of the prepare_callback option in show_in_rest .

add_action( 'init', 'wpse_89033_register_custom_meta' );
function wpse_89033_register_custom_meta() {

    $post_types = [ 

    foreach ( $post_types as $post_type ) {

                'type'              => 'string',
                'single'            => true,
                'auth_callback'     => function() { 
                    return current_user_can( 'edit_posts' );
                'show_in_rest'      => [
                    'prepare_callback' => function( $value ) {
                        return wp_json_encode( $value );



The documentation for show_in_rest (see register_meta) says it's a boolean parameter. But it will accept an options array. To see the options, look at the body of get_registered_fields, in particular the value of $default_args. The prepare_callback overrides prepare_value, the method in WP_REST_Meta_Fields that converted your custom meta value from an array to a string. With our callback, that array will instead be encoded as JSON.

On the JavaScript side, here are some key declarations.

// Get the custom meta
let customMeta = wp.data.select('core/editor').getEditedPostAttribute('meta');

// Parse the meta so you can do some JS with it
let parsed = JSON.parse(customMeta._custom_key);

// Do some JS with the parsed meta...

// Stringify the parsed meta before dispatching it
let stringified = JSON.stringify(parsed);

 * Dispatch (update) the meta (and don't forget to
 * save/publish/update the post to ensure the meta goes in the database)
wp.data.dispatch('core/editor').editPost({meta: {_custom_key: stringified}}); 

Back in PHP land, getting and updating your custom meta will now work differently. When you call get_post_meta, you're used to getting an array, and you'll probably still want one sometimes. But if you last updated your custom meta via the WP REST API, you're getting a JSON string. In that case, you'll have to use json_decode. One approach would be to wrap calls to get_post_meta like so:

function wpse_89033_get_custom_meta( $post_id, $meta_key ) {

    $post_meta = get_post_meta( $post_id, $meta_key, true );

    // If $post_meta is a string that's not empty
    if ( $post_meta && is_string( $post_meta ) ) {

         * Use the decoded JSON string or else the original string 
         * if decoding fails, e.g., if the string isn't JSON
        $post_meta = json_decode( $post_meta, true ) ?: $post_meta;


    return $post_meta;


If you make changes to your custom meta as an array, don't worry about re-encoding before calling update_post_meta. The prepare_callback will handle re-encoding.

About updating in PHP, calling update_post_meta during save_post, as shown in the official plugin handbook, appears to override updates made via the WP REST API. So one more thing to consider is conditioning any save_post update to your custom meta on whether the post is being edited in Gutenberg. If it is, leave the update to the REST API.

  • This should be the accepted answer. Thanks a lot for getting into details -- that saved my day. Cheers. Jan 13, 2020 at 15:41
  • Update: I left this solution if favor of a register_rest_field() call -- it's much more flexible. If anyone needs it, I can fill a full response describing the approach. Jan 14, 2020 at 10:02

In WordPress 5.3 you can now register meta with the array and object meta type. Here is the post from Make WordPress Core which gives all the details with examples:

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