Pouring a Drink for Everyone Who Has Had to Order By Term Slug

because-wp-query-will-not-do-this.php

<?php

add_action( 'pre_get_posts', function( $query ) {
	if ( ( is_post_type_archive( 'special-cpt' ) || ( is_search() && 'special-cpt' == $query->get( 'post_type' ) ) ) && $query->is_main_query() ) {

		$query->set( 'orderby'  , 'title' );
		$query->set( 'order'    , 'ASC' );

		$query->set( 'tax_query', array(
			array(
				'taxonomy' => 'custom_taxonomy',
				'field'    => 'slug',
				'terms'    => array( 'uno', 'dos', 'tres' ),
			),
		) );

		add_filter( 'posts_join', function( $join, $query ) {
			global $wpdb;

			return $join . ' INNER JOIN '. $wpdb->terms .' ON ( '. $wpdb->terms . '.term_id = ' . $wpdb->term_relationships . '.term_taxonomy_id )';
		}, 10, 2 );

		add_filter( 'posts_orderby', function( $join, $query ) {
			global $wpdb;

			return $wpdb->terms . '.slug,' . $wpdb->posts . '.post_title';
		}, 10, 2 );
	}
} );