Using pre_get_posts to Filter Posts

Question

I'm trying to filter archive_research.php to show only the research with status=current.

In functions.php:

add_action( 'pre_get_posts', 'only_current' );

function only_current( $query ) {
    if ( $query->is_main_query() ) {
        $args = array(
            'post_type' => 'research',
            'tax_query' => array(
                array(
                    'taxonomy' => 'status',
                    'field'    => 'slug',
                    'terms'    => 'current',
                ),
             ),
        );
        $query = new WP_Query( $args );
    }
}

In archive-research.php:

        // theloop
        if ( have_posts() ) : while ( have_posts() ) : the_post();
            // print title etc

Result: research archive displays ALL research.

I have tried many slight alterations of this. For readability I will try to sum them up here without pasting a bunch of redundant code:

• removing the tax_query .. same result (expected)
• changing the tax_query to a taxonomy term that does not exist .. same result
• replacing $query = new WP_Query( $args ) with $query->set('meta_query', $args) or $query->set($args) .. same result
• replacing have_posts() and the_post() in archive-research to $wp_query->have_posts() and $wp_query->the_post() .. same result
• adding a call to do_action('only_current') before // theloop .. same result

So the common theme here is that no matter what I do, I get the same result. There must be a very obvious solution that I'm missing, and I would really appreciate it if someone could point it out.

Last bit of debugging info that might be useful:

string(222) "SELECT SQL_CALC_FOUND_ROWS wp_posts.ID FROM wp_posts WHERE 1=1 AND wp_posts.post_type = 'research' AND (wp_posts.post_status = 'publish' OR wp_posts.post_status = 'private') ORDER BY wp_posts.post_date DESC LIMIT 0, 10"

This is the result of calling echo var_dump($wp_query->request) before // theloop

Answer 1

You're creating a new query, not altering the existing one.

The pre_get_posts codex page has a note explaining that the $query argument is passed by reference and you should alter it directly. There's no need to declare globals or return values. Update your function to be:

add_action( 'pre_get_posts', 'only_current' );
function only_current( $query ) {

    if ( is_admin() ) {
        return;
    }

    if ( ! $query->is_main_query() ) {
        return;
    }

    if ( ! is_post_type_archive( 'research' ) ) {
        return;
    }

    $tax_query = array(
        array(
            'taxonomy' => 'status',
            'field'    => 'slug',
            'terms'    => 'current',
        ),
    );
    $query->set( 'tax_query', $tax_query );
}

The pre_get_posts filter also applies to queries within the admin, so I've added another if statement to check we're on the front end of the site.

I've also added a statement to check we're on the research archive page.

The function above needs to be put in your functions.php file, or a site specific plugin. Placing it in the archive-research.php file will not work because the query will have been run by the time the archive-research.php template is loaded.