gistfile1.txt

/**
 * Implements hook_query_TAG_alter().
 */
function micro_page_query_node_access_alter(\Drupal\Core\Database\Query\SelectInterface $query) {
  // Read meta-data from query, if provided.
  if (!($account = $query->getMetaData('account'))) {
    $account = \Drupal::currentUser();
  }
  if ($account->hasPermission('bypass node access') || $account->hasPermission('link to any page')) {
    return;
  }

  $route_name = \Drupal::routeMatch()->getRouteName();

  static $active_site;
  /** @var \Drupal\micro_site\SiteNegotiatorInterface $negotiator */
  $negotiator = \Drupal::service('micro_site.negotiator');
  if (!isset($active_site)) {
    $active_site = $negotiator->getSite();
  }

  if ($active_site) {
    // @TODO support node associated to multiple sites.
    $query->condition('site_id', $active_site->id());
  }
  elseif ($route_name == 'system.entity_autocomplete') {
    $previousUrl = \Drupal::request()->server->get('HTTP_REFERER');
    $fake_request = Request::create($previousUrl);
    $url_object = \Drupal::service('path.validator')->getUrlIfValid($fake_request->getRequestUri());
    
    // blabla code to fetch the site entity from the referer route object
    $query->condition('site_id', $active_site->id());
  }

}