Add Group ID and Group Field values as classes in page.html.twig

I was able to get Group ID (and other data) into my template like so:

In theme.theme:

function theme_preprocess_page(&$variables) {

  if (($group = \Drupal::routeMatch()->getParameter('group'))) {
    $variables['group']['id'] = $group->id();
    $variables['group']['type'] = $group->getGroupType()->id();

    $styles = $group->get('field_style')->referencedEntities();
    foreach ($styles as $style) {
      $name = $style->getName();
      $variables['group']['style'] = $name;
    }

    $subjects = $group->get('field_subject')->referencedEntities();
    foreach ($subjects as $subject) {
      $name = $subject->getName();
      $variables['group']['subject'] = $name;
    }

    // Print entire Group object for testing
    //$variables['group']['object'] = $group;

  }

In page--group.html.twig:

{% if group.id %}
  {% set data_id = group.id %}
  {% set data_type = group.type %}
  {% set data_style = "#{ group.style |lower }" %}
{% endif %}

<div
  data-courseid="{{ data_id }}"
  data-coursetype="{{ data_type }}"
  data-coursestyle="{{ data_style }}"
  {{ page_attributes.addClass(page_classes)
}}>

However, now I am wondering the best approach to getting the same data attributes (or classes) in a node that is part of a group.

The following does not work:

 if (($group_relationship = \Drupal::routeMatch()->getParameter('group_relationship'))) {
  dump($group_relationship);
 }

Any suggestions?

This is how I'm pursuing adding the same sort of attributes to nodes within the group context:

if (($node = \Drupal::routeMatch()->getParameter('node'))) {

  if ($variables['node']->getType() === 'page') {
    dump($node);
    $node_id = $variables['node']->id();

    $storage = \Drupal::entityTypeManager()->getStorage('group_relationship');
    $group = $storage->loadByEntity($node);
  }
}

But, I need to figure out how to load the group entity from the group_relationship now. Or maybe there is another way.

I was able to get this work, solution shared below.

My Groups have a field called Style (term ref) and Subject (term ref). When a Group is created, these fields are selected. I add these fields as CSS classes (or data attributes) in page.html.twig, allowing me to write custom styles per "Style" or "Subject" selected.

The following hook_preprocess_page implementation adds those classes to the Group and Group nodes.

function yourtheme_preprocess_page(&$variables) {

  if (($group = \Drupal::routeMatch()->getParameter('group'))) {
    $variables['group']['id'] = $group->id();
    $variables['group']['type'] = $group->getGroupType()->id();

    $styles = $group->get('field_style')->referencedEntities();
    foreach ($styles as $style) {
      $name = $style->getName();
      $variables['group']['style'] = $name;
    }

    $subjects = $group->get('field_subject')->referencedEntities();
    foreach ($subjects as $subject) {
      $name = $subject->getName();
      $variables['group']['subject'] = $name;
    }

    // Print entire Group object for testing
    //$variables['group']['object'] = $group;

  }

  if (($node = \Drupal::routeMatch()->getParameter('node'))) {

    if ($variables['node']->getType() === 'page') {

      $node_id = $variables['node']->id();

      $storage = \Drupal::entityTypeManager()->getStorage('group_relationship');
      assert($storage instanceof GroupRelationshipStorageInterface);

      $group_relationship = $storage->loadByEntity($node);
      assert($group_relationship instanceof GroupRelationshipInterface);

      foreach ($storage->loadByEntity($variables['node']) as $group_content) {
        //$variables['group']['group'] = $group_content->getGroup();
        $variables['group']['id'] = $group_content->getGroup()->id();
        $variables['group']['type'] = $group_content->getGroupType()->id();

        $styles = $group_content->getGroup()->get('field_style')->referencedEntities();
        foreach ($styles as $style) {
          $name = $style->getName();
          $variables['group']['style'] = $name;
        }

        $subjects = $group_content->getGroup()->get('field_subject')->referencedEntities();
        foreach ($subjects as $subject) {
          $name = $subject->getName();
          $variables['group']['subject'] = $name;
        }
      }

      //dump($variables);
    }
  }
}