Stop saving an entity when it gets added to a group

Closes #2872697

src/Entity/GroupRelationship.php

@@ -2,6 +2,7 @@
 
 namespace Drupal\group\Entity;
 
+use Drupal\Core\Cache\Cache;
 use Drupal\Core\Entity\Attribute\ContentEntityType;
 use Drupal\Core\Entity\ContentEntityBase;
 use Drupal\Core\Entity\EntityChangedTrait;
@@ -24,6 +25,7 @@ use Drupal\group\Entity\Views\GroupRelationshipViewsData;
 use Drupal\group\Form\GroupJoinForm;
 use Drupal\group\Form\GroupLeaveForm;
 use Drupal\user\EntityOwnerTrait;
+use Drupal\user\UserInterface;
 
 /**
  * Defines the relationship entity.
@@ -270,11 +272,19 @@ class GroupRelationship extends ContentEntityBase implements GroupRelationshipIn
 
     if ($update === FALSE) {
       // We want to make sure that the entity we just added to the group behaves
-      // as a grouped entity. This means we may need to update access records,
-      // flush some caches containing the entity or perform other operations we
-      // cannot possibly know about. Lucky for us, all of that behavior usually
-      // happens when saving an entity so let's re-save the added entity.
-      $this->getEntity()->save();
+      // as a grouped entity. Any code that runs "live" should get the new
+      // information, but we want to invalidate cache entries that listed the
+      // entity as a cacheable dependency.
+      //
+      // One drawback to the above is that not everything gets cached. Some
+      // things get calculated and stored as entities (or other DB entries) of
+      // their own without any cacheable metadata attached. One example being
+      // the Pathauto module generating path aliases upon entity save.
+      //
+      // It is up to these modules to listen to hook_group_relationship_insert()
+      // and call ::getEntity() on the relationship to also run their code on
+      // the entity that got added to a group.
+      Cache::invalidateTags($this->getEntity()->getCacheTags());
     }
 
     // If a membership gets updated, but the member's roles haven't changed, we
@@ -305,13 +315,14 @@ class GroupRelationship extends ContentEntityBase implements GroupRelationshipIn
     foreach ($entities as $group_relationship) {
       assert($group_relationship instanceof GroupRelationshipInterface);
       if ($entity = $group_relationship->getEntity()) {
-        // For the same reasons we re-save entities that are added to a group,
-        // we need to re-save entities that were removed from one. See
-        // ::postSave(). We only save the entity if it still exists to avoid
-        // trying to save an entity that just got deleted and triggered the
-        // deletion of its relationship entities.
-        // @todo Revisit when https://www.drupal.org/node/2754399 lands.
-        $entity->save();
+        // For the same reason we invalidate the cache tags of entities that are
+        // added to a group, we need to invalidate the cache tags of those that
+        // were removed from one. See ::postSave().
+        //
+        // We can only invalidate the cache tags of entities that still exist
+        // and not the ones that were just deleted and triggered the deletion of
+        // their group relationship entities.
+        Cache::invalidateTags($entity->getCacheTags());
 
         // If a membership gets deleted, we need to reset the internal group
         // roles cache for the member in that group, but only if the user still
@@ -319,7 +330,8 @@ class GroupRelationship extends ContentEntityBase implements GroupRelationshipIn
         if ($group_relationship->getPluginId() == 'group_membership') {
           $role_storage = \Drupal::entityTypeManager()->getStorage('group_role');
           assert($role_storage instanceof GroupRoleStorageInterface);
-          $role_storage->resetUserGroupRoleCache($group_relationship->getEntity(), $group_relationship->getGroup());
+          assert($entity instanceof UserInterface);
+          $role_storage->resetUserGroupRoleCache($entity, $group_relationship->getGroup());
         }
       }
     }