<?php

/** filter to return post_ids from a multi-select, narrow results facet **/
add_filter( 'facetwp_facet_filter_posts', function( $return, $params ) {

	if ( 'product_categories' == $params['facet']['name'] ) {

		global $wpdb;

		$return          = array();
		$facet           = $params['facet'];
		$selected_values = $params['selected_values'];

		$sql = $wpdb->prepare( "SELECT DISTINCT post_id
            FROM {$wpdb->prefix}facetwp_index
            WHERE facet_name = %s",
			$facet['name']
		);

		// Match ALL values
		foreach ( $selected_values as $key => $value ) {
			$results = facetwp_sql( $sql . " AND facet_value IN ('$value')", $facet );
			$return  = ( $key > 0 ) ? array_intersect( $return, $results ) : $results;

			if ( empty( $return ) ) {
				break;
			}
		}

	}

	return $return;
}, 10, 2 );