Custom taxonomy with custom meta value is not sorting correctly (query returns the same value for orderby regardless of sort column click)

Question

I have a weight class category, where I want to sort by weight so Heavyweight shows at the top, and Flyweight at the bottom. I have created a custom meta field called 'weight' where I store an int corresponding to the weight for the respective weight class.

Here is the code where I setup the custom tax:

register_taxonomy( 'weight-class', 'ratings',
    array(
        'labels' => array(
            'name'          =>  'Weight Class',
            'singular_name' =>  'Weight Class',
            'menu_name'     =>  'Weight Classes',
            'all_items'     =>  'All Weight Classes',
            'add_new'       =>  'Add A Weight Class',
            'add_new_item'  =>  'Add New Weight Class',
            'edit_item'     =>  'Edit Weight Class',
            'view_item'     =>  'View Weight Class',
            'view_items'    =>  'View Weight Classes'
        ),
        'hierarchical' => true,
        'show_admin_column' => true
    )
);

Here is the code where I setup the custom meta data to be created and updated:

function cb_add_term_fields( $taxonomy ) {
    echo '<div class="form-field">
        <label for="weight">Maximum Weight (LBS)</label>
        <input type="text" name="weight" id="weight" />
        <p>Must be a number. Used for sorting purposes only.</p>
    </div>';
} 
add_action( 'weight-class_add_form_fields', 'cb_add_term_fields' );

function cb_edit_term_fields( $term, $taxonomy ) {
    $value = get_term_meta( $term->term_id, 'weight', true );
    echo '<tr class="form-field">
        <th>
            <label for="weight">Maximum Weight (LBS)</label>
        </th>
        <td>
            <input name="weight" id="weight" type="text" value="'.esc_attr($value).'" />
            <p class="description">Must be a number. Used for sorting purposes only.</p>
        </td>
    </tr>';
}
add_action( 'weight-class_edit_form_fields', 'cb_edit_term_fields', 10, 2 );

function cb_save_term_fields( $term_id ) {
    update_term_meta(
      $term_id,
      'weight',
      sanitize_text_field( $_POST[ 'weight' ] )
    );
  }
  add_action( 'created_weight-class', 'cb_save_term_fields' );
  add_action( 'edited_weight-class', 'cb_save_term_fields' );

Here is the code where I setup Weight as a custom sortable column:

add_filter('manage_edit-weight-class_columns', 'add_weight_column' );
function add_weight_column( $columns ){
    $columns['weight'] = __( 'Weight' );
    return $columns;
}

add_filter( 'manage_edit-weight-class_sortable_columns', 'add_weight_column_sortable' );
function add_weight_column_sortable( $sortable ){
    $sortable[ 'weight' ] = 'weight';
    return $sortable;
}

add_filter('manage_weight-class_custom_column', 'add_weight_column_content', 10, 3 );
function add_weight_column_content( $content, $column_name, $term_id ){

    if( $column_name !== 'weight' ){
        return $content;
    }

    $term_id = absint( $term_id );
    $weight = get_term_meta( $term_id, 'weight', true );

    if( !empty( $weight ) ){
        $content .= esc_attr( $weight );
    }

    return $content;
}


add_action( 'pre_get_posts', 'weight_column_orderby' );  
function weight_column_orderby( $query ) {  
    echo "<script>console.log('" . $query->get( 'orderby' ) . "');</script>";
    if( ! is_admin() )  
        return;  

    $orderby = $query->get( 'orderby' );  

    if( 'weight' == $orderby ) {  
        $query->set('meta_key','weight');  
        $query->set('orderby','meta_value_num');
    }  
} 

Everything works great until I went to test the sort function on my taxonomy. My problem currently resides with this piece of code:

add_action( 'pre_get_posts', 'weight_column_orderby' );  
function weight_column_orderby( $query ) {  
    echo "<script>console.log('" . $query->get( 'orderby' ) . "');</script>";
    if( ! is_admin() )  
        return;  

    $orderby = $query->get( 'orderby' );  

    if( 'weight' == $orderby ) {  
        $query->set('meta_key','weight');  
        $query->set('orderby','meta_value_num');
    }  
} 

As you can see, I am using a quick script tag to print out the value of orderby in the query. No matter what column I click to sort on the taxonomy list page, the script tag above always returns "menu_order title". I click on slug or description and still get this. Is there a setting I missed somewhere? Any help would be appreciated! Thanks!

Answer 1

pre_get_posts is an action hook used for modifying the arguments passed to the WP_Query class, and if you're filtering the posts in the list table at wp-admin/edit.php, then yes, you would use that hook.

However, since you're filtering the terms in the list table at wp-admin/edit-tags.php, then the hook that you should have used is pre_get_terms, which can be used to modify the arguments passed to the WP_Term_Query class.

• That class is used by get_terms() which is used by the class which renders the terms list table.

But then, I don't know for sure why ^{( maybe it's a bug in WordPress core? 🤔 )}, but when sorting by meta value, i.e. when orderby is meta_value_num, meta_value, or a key in a meta_query array, it seems we need to use the parse_term_query hook instead, which runs before pre_get_terms.

And here's an example you can try, and note that unlike WP_Query, WP_Term_Query doesn't have a get() or set() method, hence I directly accessed/modified the query_vars property:

add_action( 'parse_term_query', 'weight_column_orderby' );
function weight_column_orderby( WP_Term_Query $query ) {
    // Check whether we are at wp-admin/edit-tags.php?taxonomy=weight-class
    if ( ! is_admin() ||
        ! function_exists( 'get_current_screen' ) ||
        'edit-weight-class' !== get_current_screen()->id
    ) {
        return;
    }

    $taxonomies = (array) $query->query_vars['taxonomy'];

    // Modify the args, if `weight` is the `orderby` value, and that the query
    // is for your custom `weight-class` taxonomy.
    if ( 'weight' === $query->query_vars['orderby'] &&
        in_array( 'weight-class', $taxonomies, true )
    ) {
        $query->query_vars['meta_key'] = 'weight';
        $query->query_vars['orderby']  = 'meta_value_num';
    }
}