Shadow Taxonomy

Useful for relating Post Types to other Post Types.

Introduction

One of the hardest things to do in WordPress is creating relationships between two different post types. Often times this is accomplished by saving information about the relationships in post meta. However this leads to your code having a number of meta queries, and meta queries are generally one of the poorest, most taxing queries you can make in WordPress.

Metadata can also be a pain to keep synced. For example, when posts are deleted, what happens to the post meta you have saved in on a separate post type?

What is a Shadow Taxonomy.

A shadow taxonomy is a custom WordPress taxonomy which is created to mirror a specific post type. So anytime a post in that post type is created, updated, or deleted, the associated shadow taxonomy term is also created, updated, and deleted.

Additionally by using a taxonomy we get a nice UI of checkboxes for linking posts together for free on the post edit screen.

Useage

Step One:

Create the Shadow Taxonomy.

add_action( 'init', function() {
	register_taxonomy(
		'services-tax',
		'staff-cpt',
		array(
			'label'         => __( 'Services', 'text-domain' ),
			'rewrite'       => false,
			'show_tagcloud' => false,
			'hierarchical'  => true,
		)
	);
    // We will make our connection here in the next step.
});

Here we are simply creating a normal custom taxonomy. In our example we are creating a taxonomy to mirror a CPT we already have completed called Services. So as a convention I have named my Shadow Taxonomy 'services-tax'.

Also because I am wanting to link Services to another post type called Staff. I have registered this custom taxonomy to show up on the Staff CPT post edit screen.

Lastly, I have not made this taxonomy public. That is because I don't want anybody in there messing with the terms for this taxonomy. I want to let the Shadow Taxonomy Library handle creating, updating, and deleting the shadow taxonomy terms. That way I can ensure that my shadow taxonomy stays properly synced to it's associated post type.

Step Two:

Use the Shadow Taxonomy Library API to create an association.

\Shadow_Taxonomy\Core\create_relationship( 'service-cpt', 'service-tax' );

This one line is all you need to create the shadow taxonomy link, so that this library can kick in and take over management of the shadow taxonomy. The first argument is the custom post type name, and the second argument is the newly created shadow taxonomy name.

This line should go immediately after the register_taxonomy call in the first step.

API

get_the_posts( $post_id, $taxonomy, $cpt )

get_the_posts is a helper method provided by the library. It makes it easy to fetch the associated posts. Returns an array of WP Post Objects or false if no associated posts are found.

post_id (int) required - The ID of the post who's associations you want to find.
taxonomy (string) required - The Shadow Taxonomy Slug.
cpt (string) required - The Associated Custom Post Type Slug.

This library also contains a few WP_CLI scripts to help you manage your shadow taxonomies. The primary one is useful when you are using in this library on an existing site, which already contains a lot of posts. The following WP_CLI script will go through and create all the needed shadow taxonomy terms.

wp shadow sync --cpt=cpt_slug --tax=taxonomy_slug
// Will sync all of the input custom post types by creating terms in the input taxonomy.
wp shadow sync --cpt=cpt_slug --tax=taxonomy_slug --dry-run
// Will output a table of the changes the script will make without actually making any changes.
wp shadow sync --cpt=cpt_slug --tax=taxonomy_slug --verbose
// Will sync the terms but also output more logs as it process your data.

--cpt (string) required - The post type which you want to shadow.
--tax (string) required - The taxonomy to use as the shadow.
--dry-run (flag) optional - Will not create the term, but rather will list what changes would be made.
--verbose (flag) options - Will simply cause the script to output additional logging in running.