Index taxonomy terms related to another taxonomy

functions.php

<?php

/**
 * "Found object" is the post type
 * The post type has a "Materials" taxonomy
 * The "Materials" taxonomy is associated with a "Physical Properties" taxonomy
 *
 * For the facet's name, use "physical_properties"
 * For the facet's Data source, select the "Materials" taxonomy
 */
add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'physical_properties' == $params['facet_name'] ) {

        // TODO - create this function to return all physical properties associated with this "materials" term ID
        // It should return an array of term objects
        $physical_properties = get_physical_property_terms( $params['term_id'] );

        foreach ( $physical_properties as $prop ) {
            $params['facet_value'] = $prop->slug;
            $params['facet_display_value'] = $prop->name;
            $params['term_id'] = $prop->term_id;
            $class->insert( $params );
        }

        // Exit stage left
        $params['facet_value'] = '';
    }
    return $params;
}, 10, 2 );