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

Question

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';
    }
}