Searching Multiple Taxonomies and Custom Post Types

WordPress only passes the first to Relevanssi, Relevanssi doesn’t get to see the second taxonomy parameter. You need to create a tax_query that explains to Relevanssi what taxonomy filters you want, and how they are related to each other.

You can use relevanssi_modify_wp_query filter hook to read in the taxonomy parameters from $wp_query->query_vars, format them as a proper tax_query and then pass that tax_query to Relevanssi.

See this: https://www.relevanssi.com/knowledge-base/multiple-custom-taxonomies/

add_filter('relevanssi_modify_wp_query', 'researh_lib_tax_query');

function researh_lib_tax_query($query) {
    $tax_query = array();

    $authors = explode('+',$query->query_vars['authors']);
    $stages = explode('+',$query->query_vars['stages']);

    if (!empty($query->query_vars['authors'])) {
        $tax_query[] = array(
            'taxonomy' => 'author',
            'field' => 'slug',
            'terms' => $authors
        );
    }
    if (!empty($query->query_vars['stages'])) {
        $tax_query[] = array(
            'taxonomy' => 'development_stage',
            'field' => 'slug',
            'terms' => $stages
        );
    }
    if (!empty($tax_query)) {
        $tax_query['relation'] = 'AND'; // you can also use 'OR' here
        $query->set('tax_query', $tax_query);
    }
    return $query;
}

This is my code. I’m passing the checkboxes value to a hidden field that will be passed to the query vars. Is this correct? How will I know if this is working? And how can I use this to display search results?

Thanks

Seems correct to me. Search results will be displayed by your theme, using the search results template. This function just makes sure the taxonomy parameters are correct.

You can check that this is correct by adding this function:

add_filter('relevanssi_where', 'check_tax_query');
function check_tax_query($q) {
    var_dump($q);
    return $q;
}

If the function is working correctly, this should print out MySQL code that will restrict the search to the correct categories.