Filter not Filtering.

Turns out i had to add this to the functions.php file.

// MAKE API FILTERABLE
add_action( 'rest_api_init', 'post_meta_register' );
function post_meta_register() {
    register_api_field( 'post',
        'testing',
        array(
            'get_callback'    => 'get_post_meta',
            'update_callback' => null,
            'schema'          => null,
        )
    );
}

function get_post_meta( $object, $field_name, $request ) {
    return get_post_meta( $object[ 'id' ], $field_name, true );
}

add_filter( 'rest_query_vars', 'allow_meta' );
function allow_meta( $valid_vars ) {

        $valid_vars = array_merge( $valid_vars, array( 'meta_key', 'meta_value' ) );
        return $valid_vars;
}

Just want to add that, like Martin, the code on GitHub didn’t work for me either, but Martin’s code in his 2nd post did. Thanks Martin!

I’m using a custom post type, so if you are too, in the post_meta_register function be sure to change ‘post’ to your post type name, and of course, change ‘testing’ to the name of your ACF field.

I also had to rename the functions themselves as they were causing conflicts, but other than that everything now works as expected.

I just stumbled across this, and it took a bit of tweaking, figured I’d share what I had to do to get it working:

note the change to register_rest_field – register_api_field is deprecated.

add_action( 'rest_api_init', 'post_meta_register' );

function post_meta_register() {
    register_rest_field( 'post',
        'ACF_FIELD_NAME_HERE',
        array(
            'get_callback'    => 'get_acf_post_meta',
            'update_callback' => null,
            'schema'          => null,
        )
    );
}

function get_acf_post_meta( $object, $field_name, $request ) {
    return get_post_meta( $object[ 'id' ], $field_name, true );
}

add_filter( 'rest_query_vars', 'allow_meta' );
function allow_meta( $valid_vars ) {

        $valid_vars = array_merge( $valid_vars, array( 'meta_key', 'meta_value' ) );
        return $valid_vars;
}

Edit: I moved all of this out of functions.php and into a standalone plugin file so that I (hopefully) won’t have problems with future updates to WordPress. If you want to put this code into functions.php you may need to edit wp-settings.php to require plugin.php before functions.php so that add_action(0 and add_filter() are defined before you try to use them.