Category (Taxonomy) Dropdown Filtered By Post Type

While working on a large custom content management solution for a client I was building templates for multiple post types all sharing a common taxonomy. Most of these templates needed a dropdown to filter the results by the common taxonomy, in this case a taxonomy named “department.”

Should be fairly straight-forward, right? WordPress does offer a great function for creating taxonomy dropdowns called wp_dropdown_categories, which works with custom taxonomies as well as categories.

However, it will return all taxonomies with any posts. If you have multiple post types, you could very easily get a list of taxonomies with posts, but not the post type you want for the template – as was my issue.

Luckily, we can filter the SQL query used in the get_terms function, which is called by wp_dropdown_cateogies.

The above utilizes the term_clauses filter, and hunts for the post_type argument. The beauty of this is that we’re extending the functionality of the get_terms function directly.

This means we can use wp_dropdown_categories and simply pass our new post_type argument. Furthermore, the fallback here is the built-in function, which is a double bonus. I love the extend-ability of WordPress!

Here’s the basic usage, now that we have the filter above:

That said, I built a custom function to wrap the wp_dropdown_categories, so I could grab the current query var and add a wrapper element.

Here are some useful resources if you’re going down this rabbit hole:

• get_terms
• wp_dropdown_categories
• Understanding MySQL in WordPress
• Stack Exchange: Restrict taxonomy dropdown to post type