Filtering multiple custom fields with WP REST API 2

This solution works with get_items() in /lib/endpoints/class-wp-rest-posts-controller.php of the v2 WP Rest API.

First, you’ll want to construct the GET arguments like you would for a new WP_Query(). The easiest way to do this is with http_build_query().

$args = array (
    'filter' => array (
        'meta_query' => array (
            'relation' => 'AND',
            array (
                'key'     => 'color',
                'value'   => 'blue',
                'compare' => '=',
            ),
            array (
                'key'     => 'test',
                'value'   => 'testing',
                'compare' => '=',
            ),
        ),
    ),
);
$field_string = http_build_query( $args );

It’ll produce something like:

filter%5Bmeta_query%5D%5Brelation%5D=AND&filter%5Bmeta_query%5D%5B0%5D%5Bkey%5D=color&filter%5Bmeta_query%5D%5B0%5D%5Bvalue%5D=blue&filter%5Bmeta_query%5D%5B0%5D%5Bcompare%5D=%3D&filter%5Bmeta_query%5D%5B1%5D%5Bkey%5D=test&filter%5Bmeta_query%5D%5B1%5D%5Bvalue%5D=testing&filter%5Bmeta_query%5D%5B1%5D%5Bcompare%5D=%3D

Which, if you want readable, you can also use Chrome tools and decodeURIComponent('your-query-here') to make it a easier to read when you throw it into your JSON Rest API URL:

https://demo.wp-api.org/wp-json/wp/v2/product?filter[meta_query][relation]=AND&filter[meta_query][0][key]=color&filter[meta_query][0][value]=blue&filter[meta_query][0][compare]==&filter[meta_query][1][key]=test&filter[meta_query][1][value]=testing&filter[meta_query][1][compare]==

Note: To use your custom post type you would put product before ?

/wp-json/wp/v2/<custom-post-type>?filter[meta_query]

So you have your query but we need to instruct WP how to handle a few things:

1. Adding REST support for the custom post type product
2. Allowing the query args meta_query
3. Parsing meta_query

// 1) Add CPT Support <product>


function wpse_20160526_add_product_rest_support() {
    global $wp_post_types;

    //be sure to set this to the name of your post type!
    $post_type_name="product";
    if( isset( $wp_post_types[ $post_type_name ] ) ) {
        $wp_post_types[$post_type_name]->show_in_rest = true;
        $wp_post_types[$post_type_name]->rest_base = $post_type_name;
        $wp_post_types[$post_type_name]->rest_controller_class="WP_REST_Posts_Controller";
    }
}

add_action( 'init', 'wpse_20160526_add_product_rest_support', 25 );


// 2) Add `meta_query` support in the GET request

function wpse_20160526_rest_query_vars( $valid_vars ) {
    $valid_vars = array_merge( $valid_vars, array(  'meta_query'  ) ); // Omit meta_key, meta_value if you don't need them
    return $valid_vars;
}

add_filter( 'rest_query_vars', 'wpse_20160526_rest_query_vars', PHP_INT_MAX, 1 );


// 3) Parse Custom Args

function wpse_20160526_rest_product_query( $args, $request ) {

    if ( isset( $args[ 'meta_query' ] ) ) {

        $relation = 'AND';
        if( isset($args['meta_query']['relation']) && in_array($args['meta_query']['relation'], array('AND', 'OR'))) {
            $relation = sanitize_text_field( $args['meta_query']['relation'] );
        }
        $meta_query = array(
            'relation' => $relation
        );

        foreach ( $args['meta_query'] as $inx => $query_req ) {
        /*
            Array (

                [key] => test
                [value] => testing
                [compare] => =
            )
        */
            $query = array();

            if( is_numeric($inx)) {

                if( isset($query_req['key'])) {
                    $query['key'] = sanitize_text_field($query_req['key']);
                }
                if( isset($query_req['value'])) {
                    $query['value'] = sanitize_text_field($query_req['value']);
                }
                if( isset($query_req['type'])) {
                    $query['type'] = sanitize_text_field($query_req['type']);
                }
                if( isset($query_req['compare']) && in_array($query_req['compare'], array('=', '!=', '>','>=','<','<=','LIKE','NOT LIKE','IN','NOT IN','BETWEEN','NOT BETWEEN', 'NOT EXISTS')) ) {
                    $query['compare'] = sanitize_text_field($query_req['compare']);
                }
            }

            if( ! empty($query) ) $meta_query[] = $query;
        }

        // replace with sanitized query args
        $args['meta_query'] = $meta_query;
    }

    return $args;
}
add_action( 'rest_product_query', 'wpse_20160526_rest_product_query', 10, 2 );