Modify post filter to set custom number of posts per page and exclude child posts

Question

I have a custom post type called "galleries," which is hierarchical.

I'm using a function in my functions.php file to modify the number of posts displayed per page to 48. That part works fine, but I also need it to exclude all child posts, so that only top-level parent posts make up the 48 posts pulled in per page. Here is the current code:

    function projects_custom_number_of_posts( $query ) {
    if ( is_admin() || ! $query->is_main_query() )
        return;

    if ( is_post_type_archive( 'galleries' ) ) {
        // Display 48 posts for a custom post type called 'galleries'
        $query->set( 'posts_per_page', 48 );
        return;
    }
}
add_action( 'pre_get_posts', 'projects_custom_number_of_posts', 1 );

How can I modify the $query->set component to also limit the posts to top-level parent posts only? I tried the following, but it throws an error:

    function projects_custom_number_of_posts( $query ) {
    if ( is_admin() || ! $query->is_main_query() )
        return;

    if ( is_post_type_archive( 'galleries' ) ) {
        // Display 48 posts for a custom post type called 'galleries'
        $query->set( array ('posts_per_page'=>48, 'post_parent'=>0) );
        return;
    }
}
add_action( 'pre_get_posts', 'projects_custom_number_of_posts', 1 );

Answer 1

Just call $query->set 2 times:

$query->set('posts_per_page', 48 );
$query->set('post_parent', 0 );