Custom tax_query filter not working for Woocommerce product categories

Question

For some reason using pre_get_posts isn't filtering the products properly on the Shop page. I can tell the hook is being called on the proper page because if I print the $query variable it only shows on the page I want it to. However, the tax_query doesn't seem to be filtering properly.

The shop page is set to show categories. I would like to have it where if $_SESSION['mid'] is set, then it will only show that category on the Shop page. I know that the proper value is being passed because I can see it in the print of $query.

add_action('pre_get_posts','shop_filter_cat');

function shop_filter_cat($query) {
    if (!is_admin() && isset($_SESSION['mid']) && is_post_type_archive( 'product' ) && $query->is_main_query()) {
        $query->set('tax_query', array( // Include only products from merchant with ID = $_SESSION['mid']
            array (
                'taxonomy' => 'product_cat',
                'field' => 'slug',
                'terms' => $_SESSION['mid'],
                'operator' => 'IN'
            )
          )
        ); 
    // print_r($query);
   }
}

Answer 1

I figured out what the issue was. The page was displaying categories/subcategories and not the items themselves. This meant that woocommerce_product_subcategories() was the one filtering and that uses a different hook.

Here is what I added to my functions.php file.

add_filter( 'get_terms', 'get_subcategory_terms', 10, 3 );

function get_subcategory_terms( $terms, $taxonomies, $args ) {

  $new_terms = array();

  // if a product category and on the shop page
  if ( in_array( 'product_cat', $taxonomies ) && ! is_admin() && is_shop() && isset($_SESSION['mid'])  ) {

    foreach ( $terms as $key => $term ) {

      if ( in_array( $term->slug, array( $_SESSION['mid'] ) ) ) {
        $new_terms[] = $term;
      }

    }

    $terms = $new_terms;
  }

  return $terms;
}