Filter custom post type by custom taxomony

Question

I have several custom post types, each with one or more custom taxonomies. I'd like to add a filter for each taxonomy to the relevant custom post type edit screen, but my solution is not working.

---

The problem

First off, I know why the query is failing, but I don't know what is causing the query to be constructed in this manner.

When I inspect the $wp_global query after filtering a post type I notice that the condition AND (1=0) exists, whcih of course is never going to be true.

---

My (broken) solution

To output the various filters I use a class which can be found in this Pastebin.

To initiate the Class I'm using this code (trimmed down here for just one post type and taxonomy, whcih I will refer to for ease during the rest of this question) -

add_action('init', 'fgw_filter_taxonomies');
function fgw_filter_taxonomies(){

    new FGW_CPT_Taxonomy_Filter(
        array('file' => array('file_type'))
    );
    
}

The filters are displayed perfectly, however when using any of them WP returns no results.

---

My investigation

I have studied the resultant $wp_query global and I notice that WP is actually adding [file_type] => 22 to the query and query_var arrays, and the tax_query array appears to be created correctly.

However, as previously mentioned, WP is for some reason constructing the query incorrectly, meaning that no results are found.

[tax_query] => WP_Tax_Query Object
    (
        [queries] => Array
            (
                [0] => Array
                    (
                        [taxonomy] => file_type
                        [terms] => Array
                            (
                                [0] => 22
                            )

                        [field] => slug
                        [operator] => IN
                        [include_children] => 1
                    )

            )

        [relation] => AND
        [table_aliases:protected] => Array
            (
            )

        [queried_terms] => Array
            (
                [file_type] => Array
                    (
                        [terms] => Array
                            (
                                [0] => 22
                            )

                        [field] => slug
                    )

            )

        [primary_table] => fgw_posts
        [primary_id_column] => ID
    )

---

My question

What next? Where can I look to fix this problem? I've trawled through query.php, but I'm nore really sure what I'm looking for.

I'm genuinly stumped by this behaviour, particularly as comparing the resultant $wp_query global to that generated when filtering the builtin category or post_tag texonomy seems identical (other than the name of the taxonomy of course).

Answer 1

Ok, so the answer here is both annoying and fairly difficult to find/fix.

The wp_dropdown_categories() function outputs the dropdown for each custom taxonomy filter using the term_id as the value for each option. However, when constructing the query the WP_Query::parse_tax_query() method, by default, sets the field element to slug.

So, I had to make a custom walker and use that with the wp_dropdown_categories() function, as below -

/**
 * Custom extended FGW_Cat_Slug_Walker Class
 * Outputs the term 'slug' as the value of each option, not 'term_id' (as is default)
 */
class FGW_Cat_Slug_Walker extends Walker_CategoryDropdown{

    /**
     * Start the element output
     *
     * @param required string $output   Passed by reference. Used to append additional content
     * @param required object $category Category data object
     * @param required int    $depth    Depth of category in reference to parents. Default 0
     * @param required array  $args     An array of arguments
     * @param required int    $id       ID of the current category
     */
    public function start_el(&$output, $category, $depth = 0, $args = array(), $id = 0){

        $pad = str_repeat(' ', $depth * 3);    // Create the padding (before nested terms)

        /** Generate the HTML for this option */
        $output.= sprintf("\t".
            '<option class="%1$s" value="%2$s" %3$s>%4$s%5$s</option>',
            /** %1$s - 'class' attribute */     'level-' . $depth,
            /** %2$s - 'value' attribute */     $category->slug,
            /** %3$s - 'selected' attribute */  ($category->slug == $args['selected']) ? ' selected="selected"' : '',
            /** %4$s - option text */           $category->name,
            /** %5$s - The term count */        ($args['show_count']) ? '&nbsp;&nbsp;(' . $category->count . ')' : ''
        );

    }
}