Eager load ListPostsController needed relations (#2717)

* Eager load ListPostsController needed relations
* Add comment explaining the reason for eagerloading

src/Api/Controller/ListPostsController.php

@@ -93,6 +93,17 @@ class ListPostsController extends AbstractListController
             $results->areMoreResults() ? null : 0
         );
 
+        // Eager load discussion for use in the policies,
+        // eager loading does not affect the JSON response,
+        // the response only includes relations included in the request.
+        if (! in_array('discussion', $include)) {
+            $include[] = 'discussion';
+        }
+
+        if (in_array('user', $include)) {
+            $include[] = 'user.groups';
+        }
+
         return $results->getResults()->load($include);
     }