How to do a meta query using REST-API in WordPress 4.7+?

I am trying to filter by a meta query but no matter what I try I can not make it work.

Basically I want this query in through the rest API:

wp-json/wp/v2/books?meta_query[relation]=OR&meta_query[0][key]=arkivera&meta_query[0][value]=1&meta_query[0][compare]==

Ive tried several different custom fields but it will always return all post objects.

In my functions file I’ve added:

function api_allow_meta_query( $valid_vars ) {

  $valid_vars = array_merge( $valid_vars, array( 'meta_query') );
  return $valid_vars;
}
add_filter( 'rest_query_vars', 'api_allow_meta_query' );

I tried finishing some documentation regarding this but couldn’t, also most of the similar questions are not for wordpress 4.7 since filter[xxx] is removed.

You can write your own REST handler for custom queries if you want. In your case, the query can done by doing so:

// Register a REST route
add_action( 'rest_api_init', function () {
    //Path to meta query route
    register_rest_route( 'tchernitchenko/v2', '/my_meta_query/', array(
            'methods' => 'GET', 
            'callback' => 'custom_meta_query' 
    ) );
});

// Do the actual query and return the data
function custom_meta_query(){
    if(isset($_GET['meta_query'])) {
        $query = $_GET['meta_query'];
        // Set the arguments based on our get parameters
        $args = array (
            'relation' => $query[0]['relation'],
            array(
                'key' => $query[0]['key'],
                'value' => $query[0]['value'],
                'compare' => '=',
            ),
        );
        // Run a custom query
        $meta_query = new WP_Query($args);
        if($meta_query->have_posts()) {
            //Define and empty array
            $data = array();
            // Store each post's title in the array
            while($meta_query->have_posts()) {
                $meta_query->the_post();
                $data[] =  get_the_title();
            }
            // Return the data
            return $data;
        } else {
            // If there is no post
            return 'No post to show';
        }
    }
}

Now, all you have to do is to access:

wp-json/tchernitchenko/v2/my_meta_query?meta_query[relation]=OR&meta_query[0][key]=arkivera&meta_query[0][value]=1&meta_query[0][compare]==