only in patch2:
unchanged:
--- a/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php
@@ -1754,13 +1754,67 @@ protected function getExpectedGetRelationshipDocumentData($relationship_field_na
     }
     if (!$is_multiple) {
       $target_entity = $field->entity;
-      return is_null($target_entity) ? NULL : static::toResourceIdentifier($target_entity);
+      if (is_null($target_entity)) {
+        return NULL;
+      }
+      else {
+        $resource_identifier = static::toResourceIdentifier($target_entity);
+        $fieldStorageDefinition = $field->getFieldDefinition()
+          ->getFieldStorageDefinition();
+        $property_definition = $fieldStorageDefinition->getPropertyDefinition('target_id');
+        $is_data_reference_definition = $property_definition instanceof DataReferenceTargetDefinition;
+        if($is_data_reference_definition){
+          $resource_identifier['meta']['drupal_internal__target_id'] = is_numeric($field->target_id) ? (int) $field->target_id : $field->target_id;
+        }
+        return $resource_identifier;
+      }
     }
     else {
-      return array_filter(array_map(function ($item) {
+      $arity_counter = [];
+      $relation_list = array_filter(array_map(function ($item) use (&$arity_counter) {
+        /** @var \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem $item */
         $target_entity = $item->entity;
-        return is_null($target_entity) ? NULL : static::toResourceIdentifier($target_entity);
+
+        if (is_null($target_entity)) {
+          return NULL;
+        }
+        else {
+          $resource_identifier = static::toResourceIdentifier($target_entity);
+
+          if(!isset($arity_counter[$resource_identifier['type']])){
+            $arity_counter[$resource_identifier['type']] = [];
+            if(!isset($arity_counter[$resource_identifier['type']][$resource_identifier['id']])){
+              $arity_counter[$resource_identifier['type']][$resource_identifier['id']] = 0;
+            }
+          }
+          $arity_counter[$resource_identifier['type']][$resource_identifier['id']]++;
+
+          $fieldStorageDefinition = $item->getFieldDefinition()
+            ->getFieldStorageDefinition();
+          $property_definition = $fieldStorageDefinition->getPropertyDefinition('target_id');
+          $is_data_reference_definition = $property_definition instanceof DataReferenceTargetDefinition;
+          if ($is_data_reference_definition) {
+            $resource_identifier['meta']['drupal_internal__target_id'] = is_numeric($item->target_id) ? (int) $item->target_id : $item->target_id;
+          }
+          return $resource_identifier;
+        }
       }, iterator_to_array($field)));
+
+      $arity_map = [];
+      $relation_list = array_map(function ($identifier) use ($arity_counter, &$arity_map) {
+        if(isset($arity_counter[$identifier['type']]) && isset($arity_counter[$identifier['type']][$identifier['id']]) && $arity_counter[$identifier['type']][$identifier['id']] > 1){
+          if(!isset($arity_map[$identifier['type']])){
+            $arity_map[$identifier['type']] = [];
+            if(!isset($arity_map[$identifier['type']][$identifier['id']])){
+              $arity_map[$identifier['type']][$identifier['id']] = 0;
+            }
+          }
+          $identifier['meta']['arity'] = $arity_map[$identifier['type']][$identifier['id']]++;
+        }
+        return $identifier;
+      }, $relation_list);
+
+      return $relation_list;
     }
   }