Wordpress: pre_get_posts and order by taxonomy term - wordpress

Question

I have a category archive with a custom taxonomy "client". Now I want to order the category archive by the term of the custom taxonomy (the client name). I tried to query it by meta_value and a advanced custom taxonomy query, but I could not get it to be ordered by the name of the taxonomy term. Any suggestions to get a Wordpress loop ordered by taxonomy term name? See line $query->set( 'orderby', 'how_to_order_by_taxonomy_name' );
Registering the taxonomy
add_action( 'init', 'jp_register_project_taxonomies', 0 );
function jp_register_project_taxonomies() {
// register taxonomy to hold our clients
register_taxonomy(
'client',
array( 'post' ),
array(
'hierarchical' => false,
'public' => true,
'query_var' => true,
'rewrite' => true,
'labels' => array(
'name' => _x( 'Clients', 'taxonomy general name' ),
'singular_name' => _x( 'Client', 'taxonomy singular name' ),
'search_items' => __( 'Search Clients' ),
'all_items' => __( 'All Clients' ),
'edit_item' => __( 'Edit Client' ),
'update_item' => __( 'Update Client' ),
'add_new_item' => __( 'Add New Client' ),
'new_item_name' => __( 'New Client Name' ),
'menu_name' => __( 'Client' )
),
)
);
}
pre_get_posts Query
add_action( 'pre_get_posts', 'jp_project_taxonomy_queries' );
function jp_project_taxonomy_queries( $query ) {
if(!is_admin() && $query->is_main_query() && is_category() ):
if (get_query_var('poby') == 'client'):
$taxonomies = array();
$tax_order = (get_query_var('po') == 'DESC' ? 'DESC' : 'ASC' );
foreach (get_terms('client', array('order' => $tax_order)) as $tax ) {
$taxonomies[] = $tax->name;
}
$taxquery = array(
array(
'taxonomy' => 'client',
'terms' => $taxonomies,
'field' => 'slug',
)
);
$query->set( 'tax_query', $taxquery );
$query->set( 'orderby', 'how_to_order_by_taxonomy_name' );
endif;
endif;
}

Answer 1

As far as I know, there is no parameter to order a WP_Query by term.
My solution to this problem usually is to create a meta field that contains the term name or the term slug. This field is created when the post is saved using the hook save_post or save_post_{post_type}.
For instance, to order a list of books (post type book) by year of publication (taxonomy date) we can use this function to create the meta field:
add_action( 'save_post_book', 'prefix_save_date_as_meta', 10 );
function prefix_save_date_as_meta ($post_id) {
$years = get_the_terms($post_id,'date');
if( empty($years) )
return;
$years_list = wp_list_pluck($years,'name');
update_post_meta($post_id,'_book_date',$years_list[0]);
return;
}
And this other function to order the query:
add_filter( 'pre_get_posts', 'prefix_custom_args_for_loops' );
function prefix_custom_args_for_loops( $query ) {
if ( !is_admin() && is_post_type_archive('book') && $query->is_main_query() ) {
$query->set( 'orderby','meta_value_num');
$query->set( 'meta_key','_book_date');
}
return $query;
}