Trying to get custom post meta through Jetpack JSON API [closed]

I’m using the Jetpack for WordPress to produce a JSON API for my Events custom post type, which works fine:

https://public-api.wordpress.com/rest/v1/sites/MYSITE/posts/?type=events&number=100

However, this CPT also includes quite a few Post Meta Fields generated by Custom Meta Boxes and Fields for WordPress. These fields include things like:

• _ecmb_supporting_bands
• _ecmb_tickets_avail
• _ecmb_event_agelim

And by default these fields aren’t returned by the JSON API… I’ve tried to run queries like:

https://public-api.wordpress.com/rest/v1/sites/MYSITE/posts/?type=events&number=100&meta_key='_ecmb_supporting_bands'

Or without quotes around the meta key:
https://public-api.wordpress.com/rest/v1/sites/MYSITE/posts/?type=events&number=100&meta_key=_ecmb_supporting_bands

Unfortunately this doesn’t work. Does anyone know how I can return these custom meta fields in my JSON response?

From the documentation:

According to the JetPack JSON API docs:

By default, all metadata keys are allowed in the API, as long as they
are not protected keys. Any metadata key that starts with _ is by
default protected. Protected metadata keys can, however, be accessed
and edited by users with the edit_post_meta (used for editing and
viewing), add_post_meta and delete_post_meta capabilities as
appropriate for each operation. We’ve also added a filter
rest_api_allowed_public_metadata that allows you to specifically
whitelist certain metadata keys to be accessed by any user, even if
that key is protected.

so the rest_api_allowed_public_metadata filter is what you are looking for.

From the source code:

If you check the JetPack’s source code, you will find this part:

    function is_metadata_public( $key ) {
            if ( empty( $key ) )
                    return false;

            // whitelist of post types that can be accessed
            if ( in_array( $key, apply_filters( 'rest_api_allowed_public_metadata', array() ) ) )
                    return true;

            return false;
    }

in the file class.json-api-endpoints.php.

You can also check out the allow_bbpress_public_metadata() function here to see how to implement this rest_api_allowed_public_metadata filter.

Example:

Here is a similar example for your case:

/**
 * Whitelist protected meta keys
 *
 * @param array $allowed_meta_keys 
 * @return array $allowed_meta_keys 
 */
function custom_rest_api_allowed_public_metadata( $allowed_meta_keys )
{
    // only run for REST API requests
    if ( ! defined( 'REST_API_REQUEST' ) || ! REST_API_REQUEST )
        return $allowed_meta_keys;

    $allowed_meta_keys[] = '_ecmb_supporting_bands';
    $allowed_meta_keys[] = '_ecmb_tickets_avail';
    $allowed_meta_keys[] = '_ecmb_event_agelim';

    return $allowed_meta_keys;
}

add_filter( 'rest_api_allowed_public_metadata', 'custom_rest_api_allowed_public_metadata' );

with the JSON output similar to this one:

"metadata":[{"id":"196711","key":"_ecmb_event_agelim","value":"18"},
            {"id":"196709","key":"_ecmb_supporting_bands","value":"The Rolling Stones"}, 
            {"id":"196710","key":"_ecmb_tickets_avail","value":"5500"}]