Adding custom posts to The Loop

Hello,

Probably a silly question since I’m a newcomer to WordPress…

I have a custom posts type called post_news

The index.php of the site I’m trying to build displays the usual list of recent posts with what seems to be the usual bit (“The Loop”, apparently) :

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

I’m trying the have my custom post_news posts displayed along with the basic posts in this loop.

My best try so far, in the function.php of the theme, produces no results :

function add_news($query) {
  if ( !is_home() && $query->is_main_query() ) {
      $query->set('post_type', array( 'post', 'post_news' ) );
    }
  }

add_action('pre_get_posts','add_news');

Any advice ? Thanks !

https://wordpress.org/plugins/wck-custom-fields-and-custom-post-types-creator/