Order by taxonomy terms

order-by-taxonomy.php

<?php
add_action( 'pre_get_posts', 'w_filter_query' );
function w_filter_query( $q ) {
	if ( ! is_admin() && $q->is_main_query() && is_post_type_archive( 'recipe' ) ) {
		$q->set( 'posts_per_page', -1 );
		$q->set( 'need_special_order', 'type-recette' );
	}
}

add_filter('posts_clauses', 'w_orderby_tax_clauses', 10, 2 );
function w_orderby_tax_clauses( $clauses, $wp_query ) {
	if ( $wp_query->get( 'need_special_order' ) && '' != $wp_query->get( 'need_special_order' ) ) {
		$taxonomy = $wp_query->get( 'need_special_order' );
		global $wpdb;
		$clauses['join'] .="
			LEFT OUTER JOIN {$wpdb->term_relationships} ON {$wpdb->posts}.ID={$wpdb->term_relationships}.object_id
			LEFT OUTER JOIN {$wpdb->term_taxonomy} USING (term_taxonomy_id)
			LEFT OUTER JOIN {$wpdb->terms} USING (term_id)
			";
		$clauses['where'] .= " AND (taxonomy = '{$taxonomy}' OR taxonomy IS NULL)";
		$clauses['groupby'] = "object_id";
		$clauses['orderby'] = "GROUP_CONCAT({$wpdb->terms}.name ORDER BY name ASC) ";
		$clauses['orderby'] .= ( 'ASC' == strtoupper( $wp_query->get('order') ) ) ? 'ASC' : 'DESC';
	}
    return $clauses;
}