The WordPress custom taxonomy system allows you to create your own grouping of WordPress objects (e.g. posts, pages, and custom post types). Tags and categories, for example, are taxonomy objects that are native to the WordPress system. With the custom taxonomy framework, you can create your own tags and categories.

In the article Custom Post Type Permalinks (Part 2), Lane and Andrew asked the very good question of how we can add custom taxonomy tags to WordPress permalinks.

For example, if we have a custom taxonomy called rating we might want to set our WordPress permalink structure to -

/%rating%/%postname%

Here we consider how to insert custom taxonomy tags into the WordPress permalink structure.

1. Create a Custom Taxonomy

First, we create a custom taxonomy object called rating with the register_taxonomy WordPress function.

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add_action('init', 'my_rating_init');
 
function my_rating_init() {
	if (!is_taxonomy('rating')) {
		register_taxonomy( 'rating', 'post', 
				   array( 	'hierarchical' => FALSE, 'label' => __('Rating'),  
						'public' => TRUE, 'show_ui' => TRUE,
						'query_var' => 'rating',
						'rewrite' => true ) );
	}
}

Setting the rewrite argument to true (Line 10) will automatically add the tag %rating% to our WordPress system.

However, if we try to set our blog permalink structure to -

/%rating%/%postname%

Our new post permalinks will look like this

http://shibashake.com/wordpress-theme/%rating%/test-1

As a result we will get a 404 file not found error. This is because our %rating% tag was not properly translated.

2. Translate Our Custom Taxonomy Tag

To translate our new %rating% tag, we must hook into the permalink generation function – get_permalink.

The post_link hook allows us to translate tags for regular post objects and the post_type_link hook allows us to translate tags for custom post type objects.

Note that both filter hooks accept exactly the same arguments, so we can tie both hooks to the same function.

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add_filter('post_link', 'rating_permalink', 10, 3);
add_filter('post_type_link', 'rating_permalink', 10, 3);
 
function rating_permalink($permalink, $post_id, $leavename) {
	if (strpos($permalink, '%rating%') === FALSE) return $permalink;
 
        // Get post
        $post = get_post($post_id);
        if (!$post) return $permalink;
 
        // Get taxonomy terms
        $terms = wp_get_object_terms($post->ID, 'rating');	
        if (!is_wp_error($terms) && !empty($terms) && is_object($terms[0])) $taxonomy_slug = $terms[0]->slug;
        else $taxonomy_slug = 'not-rated';
 
	return str_replace('%rating%', $taxonomy_slug, $permalink);
}

Line 5 – If the permalink does not contain the %rating% tag, then we don’t need to translate anything.
Line 12 – Get the rating terms related to the current post object.
Line 13 – Retrieve the slug value of the first rating custom taxonomy object linked to the current post.
Line 14 – If no rating terms are retrieved, then replace our rating tag with the value not-rated.
Line 16 – Replace the %rating% tag with our custom taxonomy slug.

Our new post permalinks will look like this -

http://shibashake.com/wordpress-theme/rated-a/test-1

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