How to alter the query using pre_get_posts hook and is_post_type_archive

Question

The need: I'm trying to filter a custom post type archive using a GET parameter, so that I can split the posts in two groups depending on which terms they are tagged with.

How: So far I've decided to use the pre_get_posts hook, altering the query when the archive for this custom post type is rendered.

The problem: The code I'm using is not working. I can't seem to find the issue, but it is definitely not working. Maybe someone who has more exp with this hook may tell me what I'm missing:

function event_type_filter( $query ) {
    if ( is_post_type_archive( 'event' ) ) {

        if ( isset($_GET["event_term"]) ) {
            $tipo_de_curso = $_GET["event_term"];
            if ( $tipo_de_curso == "singleclass" ) {
                $query->set( 'cat', '1,2,3,4' );
            }
            if ( $tipo_de_curso == "course" ) {
                $query->set( 'cat', '-1,-2,-3,-4' );
            }
        }
    }
return $query;
}
add_action('pre_get_posts','event_type_filter');

Here in the Codex they recommend a similar approach, and I can't spot the differences, but sure there has to be some. There is some questions in this site about this hook that should serve as a model, but I didn't see how to adapt to my case.

Any ideas? Thanks :)

Answer 1

You should be using methods of the $query instance instead of the is_* template tags. And are the categories you are trying to filter a custom taxonomy? cat only works on the default category taxonomy:

function wpse_178500_event_type_filter( $wp_query ) {
    if ( ! empty( $_GET['event_term'] ) && $wp_query->is_main_query() && $wp_query->get( 'post_type' ) === 'event' ) {
        $tipo_de_curso = $_GET['event_term'];

        if ( ! $tax_query = $wp_query->get( 'tax_query' ) )
            $tax_query = array();

        if ( $tipo_de_curso === 'singleclass' ) {
            $tax_query[] = array(
                'taxonomy' => 'category', // Make sure this is correct! "category" is the default post category.
                'field' => 'term_id',
                'terms' => array( 1, 2, 3, 4 ),
            );

        } elseif ( $tipo_de_curso === 'course' ) {
            $tax_query[] = array(
                'taxonomy' => 'category', // Make sure this is correct! "category" is the default post category.
                'operator' => 'NOT IN',
                'field' => 'term_id',
                'terms' => array( 1, 2, 3, 4 ),
            );
        }

        $wp_query->set( 'tax_query', $tax_query );
    }
}

add_action( 'pre_get_posts','wpse_178500_event_type_filter' );