added ability to specify groups in reference

Rest/Metadata/Reference.php

@@ -16,4 +16,27 @@ final class Reference
      * @var string
      */
     public $field = 'id';
-}
+
+    /**
+     * @var string[]
+     */
+    public $groups;
+
+    /**
+     * @param array $data
+     *
+     * @throws InvalidArgumentException
+     */
+    public function __construct(array $data)
+    {
+        $groups = isset($data['groups']) ? (array) $data['groups'] : [];
+
+        foreach ($groups as $group) {
+            if (!\is_string($group)) {
+                throw new InvalidArgumentException(sprintf('Parameter of annotation "%s" must be a string or an array of strings.', \get_class($this)));
+            }
+        }
+
+        $this->groups = $groups;
+    }
+}

Serializer/Normalizer/AttributeExtractionStrategy.php

@@ -65,6 +65,12 @@ public function getValue($object, $attribute, $format = null, array $context = a
 
         $property = $referenceMetadata->field;
 
+        if (isset($context['groups']) && is_array($context['groups'])) {
+            if (count(array_intersect($context['groups'], $referenceMetadata->groups)) > 0) {
+                return $propertyValue;
+            }
+        }
+
         if ($propertyValue instanceof \Traversable) {
             $value = [];
 
@@ -97,4 +103,4 @@ private function isExpand($attribute, $context)
 
         return false;
     }
-}
+}