Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/serialization/src/EntityResolver/TargetIdResolver.php

@@ -0,0 +1,27 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\serialization\EntityResolver\TargetIdResolver.
+ */
+
+namespace Drupal\serialization\EntityResolver;
+
+use Symfony\Component\Serializer\Normalizer\NormalizerInterface;
+
+/**
+ * Resolves entities from data that contains an entity target ID.
+ */
+class TargetIdResolver implements EntityResolverInterface {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resolve(NormalizerInterface $normalizer, $data, $entity_type) {
+    if (isset($data['target_id'])) {
+      return $data['target_id'];
+    }
+    return NULL;
+  }
+
+}