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

Use filter rest_{CUSTOM_POST_TYPE}_query to add the post meta support in WP Rest API for any post type.

Check the Gist search post by post meta with rest API.

How to use?

http://example.org/wp-json/wp/v2/post?meta_key=<my_meta_key>&meta_value=<my_meta_value>

Example

Get posts which post meta already-visited value is true.

http://example.org/wp-json/wp/v2/post?meta_key=already-visited&meta_value=true

It ONLY lists the posts which post meta key already-visited have value true.

For reference Search post by post meta with rest api.

Available Filters

• rest_post_query
• rest_page_query
• rest_attachment_query

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]==

The following code should add multiple meta queries capability to all your post types. It supports CPT (Custom Post Type) and ACF (Advanced Custom Field). Source code also available on Github.

Add it to your function.php

add_action( 'rest_api_init', 'wp_rest_filter_add_filters' );
 /**
  * Add the necessary filter to each post type
  **/
function wp_rest_filter_add_filters() {
    foreach ( get_post_types( array( 'show_in_rest' => true ), 'objects' ) as $post_type ) {
        add_filter( 'rest_' . $post_type->name . '_query', 'wp_rest_filter_add_filter_param', 10, 2 );
    }
}
/**
 * Add the filter parameter
 *
 * @param  array           $args    The query arguments.
 * @param  WP_REST_Request $request Full details about the request.
 * @return array $args.
 **/
function wp_rest_filter_add_filter_param( $args, $request ) {
    // Bail out if no filter parameter is set.
    if ( empty( $request['filter'] ) || ! is_array( $request['filter'] ) ) {
        return $args;
    }
    $filter = $request['filter'];
    if ( isset( $filter['posts_per_page'] ) && ( (int) $filter['posts_per_page'] >= 1 && (int) $filter['posts_per_page'] <= 100 ) ) {
        $args['posts_per_page'] = $filter['posts_per_page'];
    }
    global $wp;
    $vars = apply_filters( 'rest_query_vars', $wp->public_query_vars );
    function allow_meta_query( $valid_vars )
    {
        $valid_vars = array_merge( $valid_vars, array( 'meta_query', 'meta_key', 'meta_value', 'meta_compare' ) );
        return $valid_vars;
    }
    $vars = allow_meta_query( $vars );

    foreach ( $vars as $var ) {
        if ( isset( $filter[ $var ] ) ) {
            $args[ $var ] = $filter[ $var ];
        }
    }
    return $args;
}

IMHO, a more better way would to include the additional function as a seperate plugin. So even when user changed theme, your api calls won't be affected.

Therefore I've developed a plugin for meta query in WordPress. Better yet, it supports ACF too!

WP REST Filter

After installation, just do a GET request in the following format.

https://domain.com/wp-json/acf/v3/customposttype?filter[meta_key]=acfkey&filter[meta_value]=acfvalue

Check out the plugin source code from Github.