FacetWP – indexer Reviewer plugin data

<?php

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'rating' == $params['facet_name'] ) {
        $post_id = (int) $params['post_id'];

        /**
         * IMPORTANT
         * he following line needs editing. How to grab the average rating via the post ID?
         */
        $average_rating = get_the_average_rating( $post_id );

        $params['facet_value'] = $average_rating;
        $params['facet_display_value'] = $average_rating;
    }
    return $params;
}, 10, 2 );