How do I programmatically retrieve the parent group from a subgroup?

Well, this seems to work, though it's not through any kind of Subgroup API. I'm guessing there is probably a better solution through the Supgroup API, but for now I'm using this:

$parent_relationships = GroupRelationship::loadByEntity($group);
  if (!empty($parent_relationships) && $parent_relationship = array_pop($parent_relationships)) {
    $parent = $parent_relationship->getGroup();
  }
}

This works for me, thanks @jaypan

I have found that

$parent_entity = $this->entityTypeManager->getHandler('group', 'subgroup')->getParent($child_entity);

works. It has been quite a challenge to find out. :-)

I have found that

$parent_entity = $this->entityTypeManager->getHandler('group', 'subgroup')->getParent($child_entity);

works. It has been quite a challenge to find out. :-)

That works for me, thanks! That does seem a better method.

$parent_entity = $this->entityTypeManager->getHandler('group', 'subgroup')->getParent($child_entity);

doesn't work for me. It throws MalformedLeafExceptions, at least for subgroups added programmatically.

That may potentially indicate you haven't added the subgroup correctly when doing it programatically. I would start by doing a comparison of one added through the UI, and one added programatically, to make sure you haven't missed something.

@Jaypan, that sounds legit. More helpful would be a proper documentation with examples how to do things (adding, removing, ...) subgroups correctly via API.

Simplified example of how I added lots of cases (subgroup) to projects (group and subgroup v.1):

	$group_creator = $some_uid;
	$group_creator_account = \Drupal\user\Entity\User::load($group_creator);
	$group = \Drupal::entityTypeManager()->getStorage('group')->create(['type' => 'case']);
	$group->setOwner($group_creator_account);
	$group->set('label', $somelabel);
	$group->set("status", 1);
	$group->save();
	$caseid = $group->id();
	$pluginId = 'subgroup:case';
	$project = \Drupal\group\Entity\Group::load($projectid);
	$relation = $project->getContentByEntityId($pluginId, $caseid);
	if (!$relation) {
		$project->addContent($group, $pluginId);
	}
	$project->save();