Order WordPress posts based on their taxonomy name.

order-posts-by-taxonomy.php

<?php

use WP_Query;

/**
 * Class OrderPostsByTaxonomy
 *
 * @link https://scribu.net/wordpress/sortable-taxonomy-columns.html
 *
 * Handles the ordering of posts based on their associated taxonomy.
 */
class OrderPostsByTaxonomy {

	/**
	 * The taxonomy used for ordering posts.
	 *
	 * @var string
	 */
	private $taxonomy;

	/**
	 * Constructor.
	 *
	 * @param string $taxonomy The taxonomy to use for ordering posts.
	 */
	public function __construct( string $taxonomy ) {
		$this->taxonomy = $taxonomy;
		add_filter( 'posts_clauses', array( $this, 'posts_clauses' ), 10, 2 );
	}

	/**
	 * Modifies the SQL query clauses for custom post ordering.
	 *
	 * @param array    $clauses The list of clauses for the query.
	 * @param WP_Query $query   The WP_Query instance.
	 * @return array Modified clauses.
	 */
	public function posts_clauses( array $clauses, WP_Query $query ) {
		global $wpdb;

		$orderby = $query->get( 'orderby' );

		if ( $this->taxonomy === $orderby || ( is_array( $orderby ) && array_key_exists( $this->taxonomy, $orderby ) ) ) {

			$prefix = str_replace( array( ' ', '-' ), '_', $this->taxonomy );
			$tr     = esc_sql( "{$prefix}_term_relationships" );
			$tt     = esc_sql( "{$prefix}_term_taxonomy" );
			$t      = esc_sql( "{$prefix}_terms" );

			$clauses['join'] .= <<<SQL
LEFT OUTER JOIN {$wpdb->term_relationships} AS {$tr} ON {$wpdb->posts}.ID={$tr}.object_id
LEFT OUTER JOIN {$wpdb->term_taxonomy} AS {$tt} ON {$tr}.term_taxonomy_id={$tt}.term_taxonomy_id
LEFT OUTER JOIN {$wpdb->terms} AS {$t} ON {$tt}.term_id={$t}.term_id
SQL;

			$clauses['where']   .= " AND (taxonomy = '{$this->taxonomy}' OR taxonomy IS NULL)";
			$clauses['groupby']  = "{$tr}.object_id";
			$clauses['orderby']  = "GROUP_CONCAT({$t}.name ORDER BY name ASC) ";
			$clauses['orderby'] .= ( 'ASC' === strtoupper( $query->get( 'order' ) ) ) ? 'ASC' : 'DESC';

		}

		return $clauses;
	}
}