Sharing taxonomies across different post types – like having one set of “Categories” or “Tags” work for both your blog posts and your custom “Products” post type – is a common need for many WordPress sites. The good news is, it’s definitely doable, and when done right, it can save you a ton of hassle and keep your content organized more effectively. Let’s dig into how you can actually make this happen, and more importantly, how to do it without shooting yourself in the foot later.
At its heart, sharing taxonomies means telling WordPress that a specific taxonomy (like “Categories”) isn’t just for one type of content (like “Posts”), but for multiple types. This might sound simple, but the “how” depends on how you’re setting things up. Are you using built-in WordPress features, or are you diving into code?
When Does This Even Come Up?
You might find yourself wanting to share taxonomies in a few common scenarios:
- E-commerce: You have “Products” and want to tag them with “Seasons” just like you might tag blog posts with “Seasons.”
- Portfolio Sites: You have “Projects” and “Case Studies,” and you want both to be filterable by “Industry” or “Service Type.”
- Membership Sites: You have “Courses” and “Tutorials,” and you want a unified way to categorize them by “Subject.”
- Bridging Content: You have “Blog Posts” and a custom post type for “Events,” and you want to assign them to the same “Location” taxonomy.
The goal is usually to create a more unified and manageable system for content organization. Instead of having “Product Categories” and “Blog Categories,” you have one overarching “Categories” that applies to both.
If you’re looking to deepen your understanding of sharing taxonomies across multiple post types, you might find the article on this topic particularly helpful. It provides insights and practical tips that can enhance your WordPress development skills. For more information, you can check out the article at this link.
Method 1: The Built-in WordPress Way (Registering Taxonomies with Multiple Post Types)
This is the most straightforward and recommended method if you’re comfortable registering custom post types and taxonomies. Even if you’re not a full-blown developer, many plugins make this process visually accessible.
Understanding register_taxonomy()
WordPress’s powerful register_taxonomy() function is the key here. When you register a taxonomy, you specify which post types it should be associated with. You can assign a single taxonomy to one or more post types in this registration.
Here’s a simplified PHP example showing how it’s done:
“`php
// In your theme’s functions.php or a custom plugin file
// Register a custom taxonomy called ‘service_type’
function register_custom_service_type_taxonomy() {
$labels = array(
‘name’ => _x( ‘Service Types’, ‘taxonomy general name’ ),
‘singular_name’ => _x( ‘Service Type’, ‘taxonomy singular name’ ),
‘search_items’ => __( ‘Search Service Types’ ),
‘all_items’ => __( ‘All Service Types’ ),
‘parent_item’ => __( ‘Parent Service Type’ ),
‘parent_item_colon’ => __( ‘Parent Service Type:’ ),
‘edit_item’ => __( ‘Edit Service Type’ ),
‘update_item’ => __( ‘Update Service Type’ ),
‘add_new_item’ => __( ‘Add New Service Type’ ),
‘new_item_name’ => __( ‘New Service Type Name’ ),
‘menu_name’ => __( ‘Service Types’ ),
);
$args = array(
‘hierarchical’ => true, // Set to false for tags (flat structure)
‘labels’ => $labels,
‘show_ui’ => true,
‘show_admin_column’ => true,
‘query_var’ => true,
‘rewrite’ => array( ‘slug’ => ‘service-type’ ),
// This is the crucial part: associating with multiple post types
‘associated_post_types’ => array( ‘post’, ‘project’, ‘case_study’ ),
);
register_taxonomy( ‘service_type’, array( ‘post’, ‘project’, ‘case_study’ ), $args );
}
add_action( ‘init’, ‘register_custom_service_type_taxonomy’, 0 );
“`
Breaking Down the Key Parameter: associated_post_types
In the example above, the array passed as the second argument to register_taxonomy (or the associated_post_types argument if you’re using a more structured approach on newer WordPress versions) is where the magic happens. It’s a list of slugs for the post types you want this taxonomy to be available for. In this case, 'post', 'project', and 'case_study'.
If you’re managing this through a plugin like CPT UI (Custom Post Type UI) or ACF (Advanced Custom Fields) for custom taxonomies, you’ll typically find a setting during taxonomy creation where you can select which post types it should be associated with.
Advantages of This Method:
- Clean Implementation: This is the “correct” WordPress way. It uses the core functions and is well-supported.
- Future-Proof: As long as you’re using standard WordPress or well-coded plugins, this approach will continue to work.
- Full WordPress Integration: The shared taxonomy will appear naturally in the WordPress admin interface for each associated post type. You’ll see it in the post editor, and you can query it just like any other taxonomy.
- Easier Queries: When you query for posts, you can easily filter by terms in this shared taxonomy, and it will pull results from all associated post types.
What About Existing Built-in Taxonomies?
WordPress has built-in taxonomies like “Categories” and “Tags” that are initially associated only with the “post” post type. You can actually re-associate them with other post types using the same register_taxonomy() function.
Imagine you have a custom post type called “events.” You might want your existing “Categories” to work for both blog posts and events.
“`php
// In your theme’s functions.php or a custom plugin file
function associate_categories_with_events() {
// Re-associate ‘category’ taxonomy with the ‘event’ post type.
// This needs to be done after the ‘event’ post type is registered.
register_taxonomy_for_object_type( ‘category’, ‘event’ );
}
add_action( ‘init’, ‘associate_categories_with_events’ );
“`
Explanation:
This code snippet uses register_taxonomy_for_object_type(). This function is specifically designed to attach an existing taxonomy (like ‘category’ or ‘post_tag’) to a new or existing post type.
Caveats:
- Potential Conflicts: While this works, be cautious. WordPress expects many things to work uniformly with ‘category’ and ‘post_tag’. If you have advanced functionality or plugins that heavily rely on these taxonomies only being for posts, you might encounter subtle issues.
- Order of Operations: Ensure your custom post type is registered before you try to re-associate the taxonomy. The
inithook is generally reliable.
Method 2: Plugin Solutions for Easier Management
If you’re not keen on writing PHP code, or if you prefer a visual interface for managing your post types and taxonomies, plugins are your best friend.
Custom Post Type UI (CPT UI)
CPT UI is a very popular and user-friendly plugin for creating custom post types and custom taxonomies. When you create a new taxonomy with CPT UI, it provides a clear interface to select which post types the taxonomy should be attached to.
How to Use CPT UI:
- Install and activate CPT UI.
- Go to CPT UI > Add/Edit Taxonomies.
- Fill in the basic details for your taxonomy (name, singular label, plural label).
- In the “Attach to Post Type(s)” section, you’ll see a list of all registered post types (including built-in ones like Posts).
- Check the boxes for all the post types you want this taxonomy to apply to.
- Click “Add Taxonomy.”
Advantages of CPT UI:
- No Coding Required: Completely visual and user-friendly.
- Clear Interface: The “Attach to Post Type(s)” option is straightforward.
- Widely Used: It integrates well with many other plugins.
Advanced Custom Fields (ACF)
ACF is a powerhouse for adding custom fields, but it also excels at creating custom post types and taxonomies, especially when you want more control over field placement and display. When setting up a custom taxonomy in ACF, you’ll also have an option to choose its associated post types.
How to Use ACF:
- Install and activate ACF (Pro is recommended for these features).
- Go to Custom Fields > Taxonomies.
- Click “Add New Taxonomy.”
- Configure your taxonomy settings.
- Under the “Post Types” section, select the post types you want this taxonomy to be available for.
- Click “Save Changes.”
Advantages of ACF:
- Integrated Solution: If you’re already using ACF for custom fields, managing taxonomies here keeps everything in one place.
- Powerful Options: Offers more granular control over taxonomy behavior and settings.
Other Taxonomy Management Plugins
There are other plugins out there that offer similar functionality, often with slightly different interfaces or feature sets. Some might focus exclusively on taxonomies, while others are broader site management tools. If you’re looking for a plugin, search the WordPress repository for “custom taxonomy” or “taxonomy management.”
Key Takeaway for Plugins:
The main benefit of using a plugin like CPT UI or ACF is that they handle the underlying register_taxonomy() calls for you, presenting a user-friendly interface to achieve the same result.
Method 3: The “Less Ideal” but Sometimes Necessary Approach (Term Synchronization)
This isn’t technically sharing a taxonomy in the WordPress sense, but it’s a workaround that some people use if they’re stuck with separate taxonomies and need to keep their terms in sync. This usually involves custom code or a dedicated plugin to copy terms from one taxonomy to another, or to update terms across different post types when one is changed.
Why this is less ideal:
- Data Duplication: You’re essentially creating the same set of terms in multiple, separate taxonomies. This can lead to inconsistencies and make management harder.
- Increased Complexity: You need code or a plugin to manage the synchronization, which adds another layer of maintenance.
- Querying Issues: If you want to find all items belonging to “Category X,” you might have to query for both “post\_category X” and “product\_category X,” which is cumbersome.
When You Might Consider This (and why it’s usually a last resort):
- Legacy Systems: You have an existing site with separate taxonomies (e.g., “Product Categories” for products, “Post Categories” for posts) and rewriting the entire structure is prohibitive.
- Third-Party Integrations: A specific integration requires terms to exist in distinct taxonomies, and you can’t modify that integration.
How it Might Work (Conceptual – Requires Custom Code):
You’d typically hook into actions like save_post or edit_terms. When a term is saved to taxonomy A, your code would check if a corresponding term exists in taxonomy B and create it, or update it if it already does. This can get complicated quickly, especially with hierarchical taxonomies.
Example (Very Basic Concept – Not Production Ready):
“`php
// This is a simplified concept.
// Real-world implementation would need error handling,
// checking for existing terms, handling hierarchies, etc.
function sync_product_cat_to_post_cat( $term_id, $tt_id, $taxonomy ) {
if ( ‘product_category’ === $taxonomy ) {
$term = get_term( $term_id, $taxonomy );
if ( $term && ! is_wp_error( $term ) ) {
$existing_term = term_exists( $term->name, ‘category’ );
if ( $existing_term === 0 ) { // Term doesn’t exist
wp_insert_term( $term->name, ‘category’ );
} else {
// Optionally update existing term properties if needed
}
}
}
}
add_action( ‘created_term’, ‘sync_product_cat_to_post_cat’, 10, 3 );
add_action( ‘edited_term’, ‘sync_product_cat_to_post_cat’, 10, 3 );
“`
Strong Recommendation: Avoid this method if at all possible. Focus on Method 1 or 2 to set up your taxonomies correctly from the start.
When exploring the intricacies of sharing taxonomies across multiple post types, it’s beneficial to consider best practices that can enhance your WordPress site’s organization and functionality. A related article that delves deeper into this topic can be found at The Sheryar Blog, where you can discover additional insights and strategies for effectively managing taxonomies. This resource will help you ensure that your content is well-structured and easily navigable, ultimately improving user experience and SEO performance.
Method 4: Displaying and Querying Shared Taxonomies
Once you’ve got a taxonomy shared across post types, you’ll want to display and query it effectively.
In the WordPress Admin Area
- Post/Page Editor: When editing a post or page that’s associated with the shared taxonomy, you’ll see the taxonomy metabox appear in the sidebar (or wherever you’ve placed it). You can add, remove, or select terms just like you would for a single-post-type taxonomy.
- Admin Columns: If you enabled
show_admin_column(or if your plugin supports it), you’ll see columns for your shared taxonomy in the list view of each associated post type. This is incredibly helpful for quick sorting and identification.
Front-End Display and Filtering
This is where the real benefit shines. You can create archives and filters that pull from all associated post types.
Querying for Posts with Shared Taxonomies:
When you construct a WordPress query (using WP_Query), you can specify terms from your shared taxonomy.
“`php
$args = array(
‘post_type’ => array( ‘post’, ‘project’, ‘case_study’ ), // Search across multiple post types
‘tax_query’ => array(
array(
‘taxonomy’ => ‘service_type’, // Your shared taxonomy slug
‘field’ => ‘slug’,
‘terms’ => ‘web-design’, // The term slug you’re looking for
),
),
);
$query = new WP_Query( $args );
if ( $query->have_posts() ) :
while ( $query->have_posts() ) : $query->the_post();
// Display post title, content, etc.
the_title();
the_excerpt();
endwhile;
wp_reset_postdata();
else :
echo ‘No posts found.’;
endif;
?>
“`
Creating Filterable Archives:
You can build archive pages or sidebar widgets that allow users to filter content by shared taxonomy terms. To do this, you’d typically:
- Fetch all terms for your shared taxonomy.
- Display these terms as links (e.g., in a list or dropdown).
- When a term link is clicked, pass the taxonomy and term slug as URL parameters (e.g.,
?service_type=web-design). - In your archive template, check for these URL parameters and modify your
WP_Queryaccordingly.
Example of Building Filter Links:
“`php
$terms = get_terms( array(
‘taxonomy’ => ‘service_type’, // Your shared taxonomy
‘hide_empty’ => true,
) );
if ( ! empty( $terms ) && ! is_wp_error( $terms ) ) {
echo ‘
- ‘;
- ‘ . esc_html( $term->name ) . ‘
foreach ( $terms as $term ) {
$term_link = add_query_arg( ‘service_type’, $term->slug, get_post_type_archive_link( ‘post’ ) ); // Or your main archive page
echo ‘
‘;
}
echo ‘
‘;
}
?>
“`
Displaying Terms on Individual Posts
You might also want to display the terms associated with a post on the front-end.
“`php
// Inside the WordPress Loop
$terms = get_the_terms( get_the_ID(), ‘service_type’ ); // Your shared taxonomy
if ( $terms && ! is_wp_error( $terms ) ) {
echo ‘
Service Types: ‘;
$term_links = array();
foreach ( $terms as $term ) {
// Link to the archive page for this term and post type
$term_link = get_term_link( $term, ‘service_type’ );
$term_links[] = ‘‘ . esc_html( $term->name ) . ‘‘;
}
echo implode( ‘, ‘, $term_links );
echo ‘
‘;
}
?>
“`
Method 5: Considerations and Best Practices
Sharing taxonomies is powerful, but it’s important to do it thoughtfully.
Naming Conventions
Choose clear and descriptive names for your taxonomies. If a taxonomy is intended to apply to multiple post types, its name should reflect that broader purpose. For instance, instead of “Product Categories” and “Blog Categories,” use “Categories” or “Content Categories” if they are truly interchangeable.
Hierarchical vs. Flat Taxonomies
- Hierarchical (Categories): Excellent for structured, tree-like organization. Think of a product catalog with main categories and subcategories.
- Flat (Tags): Good for providing descriptive keywords that don’t necessarily need a strict hierarchy.
Decide which structure best suits the kind of organization you need for your shared taxonomy.
Performance Impact
Sharing taxonomies is generally efficient. WordPress is built to handle this. However, if you have an enormous number of terms and an extremely high volume of content across many post types, query optimization might become more critical. Properly indexed database tables and efficient queries (like the examples above) are key.
Plugin Conflicts
While most well-coded plugins play nicely together, always test thoroughly, especially after adding new plugins or making significant changes. If you encounter issues, try deactivating other plugins one by one to pinpoint the culprit. If the problem stems from your custom theme code, consider if it might conflict with a plugin’s output.
User Experience (UX) in the Admin
Ensure that the taxonomy is displayed in a logical place in the admin editor. Sometimes plugins override default metabox placement; if you find the taxonomy box is hard to find, look for options within your plugins or consider custom code to adjust its position.
Future Maintainability
When setting up shared taxonomies, think about how easy it will be to manage them down the line. Using Method 1 (code) or Method 2 (plugins like CPT UI or ACF) ensures that your taxonomy registration is clear and documented, whether in your code or within the plugin’s interface. This makes it easier for you or another developer to understand and modify in the future.
Conclusion
Sharing taxonomies across multiple post types in WordPress is a robust feature that, when implemented correctly, significantly enhances content organization and user navigation. The most reliable and recommended approach is to define your taxonomy and specify all the post types it should be associated with during its registration, either through code using register_taxonomy() or via user-friendly plugins like CPT UI or ACF. While workarounds exist, they generally introduce complexity and are best avoided. By following these guidelines, you can create a more unified, efficient, and manageable WordPress site.