only call serialize once for better performance

src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php

@@ -371,7 +371,9 @@ private function isMaxDepthReached(array $attributesMetadata, $class, $attribute
     }
 
     /**
-     * Overwrite to update the cache key for the child.
+     * Overwritten to update the cache key for the child.
+     *
+     * We must not mix up the attribute cache between parent and children.
      *
      * {@inheritdoc}
      */
@@ -385,7 +387,7 @@ protected function createChildContext(array $parentContext, $attribute)
     }
 
     /**
-     * Build the cache key for the attributes cache.
+     * Builds the cache key for the attributes cache.
      *
      * The key must be different for every option in the context that could change which attributes should be handled.
      *
@@ -397,7 +399,11 @@ protected function createChildContext(array $parentContext, $attribute)
     private function getAttributeCacheKey($format, array $context)
     {
         try {
-            return md5($format.serialize($context).serialize($this->ignoredAttributes)).serialize($this->camelizedAttributes);
+            return md5($format.serialize([
+                'context' => $context,
+                'ignored' => $this->ignoredAttributes,
+                'camelized' => $this->camelizedAttributes,
+            ]));
         } catch (\Exception $exception) {
             // The context cannot be serialized, skip the cache
             return false;