diff --git a/src/SeriousJSON/IJsonUnflattener.php b/src/SeriousJSON/IJsonUnflattener.php
index b92baf7..a95ac4c 100644
--- a/src/SeriousJSON/IJsonUnflattener.php
+++ b/src/SeriousJSON/IJsonUnflattener.php
@@ -12,4 +12,6 @@ interface IJsonUnflattener
      * @return JsonSerializable|null
      */
     public function unflattenByID(string $className, $identifier) : JsonSerializable|null;
+
+    public function updateIndex(IJsonIdentifiable $source, IJsonIdentifiable $dest, bool $delete);
 }
\ No newline at end of file
diff --git a/src/SeriousJSON/JsonDatabase.php b/src/SeriousJSON/JsonDatabase.php
index a260c21..0395ad1 100644
--- a/src/SeriousJSON/JsonDatabase.php
+++ b/src/SeriousJSON/JsonDatabase.php
@@ -8,6 +8,7 @@ class JsonDatabase implements IJsonUnflattener
     private static $LIVE = 'live';
     private static $ARCHIVE = 'archive';
     private static $EXTENSION = '.json';
+    private static $EXTENSION_INDEX = '.index';
     private ?string $storagePath = null;
     private bool $nested = false;
     private bool $fullDomain = true;
@@ -148,6 +149,87 @@ class JsonDatabase implements IJsonUnflattener
             return JsonSerializable::Deserialize($json, $this->nested, $this, $class);
     }
 
+    public function updateIndex(IJsonIdentifiable $source, IJsonIdentifiable $dest, bool $delete = false)
+    {
+        if ($source->flatIdentifier() == null || $dest->flatIdentifier() == null)
+            return;
+
+        $destID = self::SanitizeIdentifier(strval($dest->flatIdentifier()));
+        $srcID = self::SanitizeIdentifier(strval($source->flatIdentifier()));
+
+        if (empty($destID) || empty($srcID))
+            return;
+
+        $destPath = $this->getObjectPath(get_class($dest), $destID) . self::$EXTENSION_INDEX;
+
+        if (strlen($destPath) > PHP_MAXPATHLEN)
+            throw new Exception('JsonDatabase: Specified Path ' . $destPath . ' is longer than allowed.');
+
+        $indexJson = [];
+        // Index exists
+        if (is_file($destPath))
+        {
+            $indexJson = json_decode(file_get_contents($destPath), true);
+
+            if ($indexJson == false)
+                $indexJson = [];
+        }
+
+        // Check if entry exists
+        $subArray = [];
+        if (array_key_exists(get_class($source), $indexJson))
+            $subArray = $indexJson[get_class($source)];
+        
+        // Already indexed
+        if (in_array($srcID, $subArray) && !$delete)
+            return;
+        else if (in_array($srcID, $subArray) && $delete)
+            unset($subArray[array_search($srcID, $subArray)]);
+        else
+            $subArray[] = $srcID;
+
+        $indexJson[get_class($source)] = $subArray;
+
+        if (is_writable(dirname($destPath)) || mkdir(dirname($destPath), 0770, true))
+            file_put_contents($destPath, json_encode($indexJson));
+        else
+            throw new \Exception('JsonDatabase: Path ' . $destPath . ' is not writable.');
+    }
+
+    public function fromIndex(string $serClass, $serIdentifier, string $findClass): array
+    {
+        if ($serIdentifier == null)
+            return [];
+        
+        $serIdentifier = self::SanitizeIdentifier(strval($serIdentifier));
+        if (empty($serIdentifier) || empty($serClass))
+            return [];
+
+        $serPath = $this->getObjectPath($serClass, $serIdentifier) . self::$EXTENSION_INDEX;
+
+        if (strlen($serPath) > PHP_MAXPATHLEN)
+            throw new Exception('JsonDatabase: Specified Path ' . $destPath . ' is longer than allowed.');
+
+        $indexJson = [];
+        // Index exists
+        if (is_file($serPath))
+        {
+            $indexJson = json_decode(file_get_contents($serPath), true);
+
+            if ($indexJson == false)
+                return [];
+
+            // TODO: Check if the referenced objects exist on disk
+            if (array_key_exists(get_class($findClass), $indexJson))
+                return $indexJson[get_class($findClass)];
+            else
+                return [];
+        }
+        else {
+            return [];
+        }
+    }
+
     public function save(JsonSerializable $obj, ?string $name = null)
     {
         if ($name != null && !empty(trim($name))) {
@@ -184,7 +266,7 @@ class JsonDatabase implements IJsonUnflattener
         if (is_writable(dirname($objPath)) || mkdir(dirname($objPath), 0770, true))
             file_put_contents($objPath, $objJson);
         else
-            throw new \Exception('JsonDatabase: Path ' . $historyPath . ' is not writable.');
+            throw new \Exception('JsonDatabase: Path ' . $objPath . ' is not writable.');
         
         return $name;
     }
diff --git a/src/SeriousJSON/JsonSerializable.php b/src/SeriousJSON/JsonSerializable.php
index 41e6e60..bba1835 100644
--- a/src/SeriousJSON/JsonSerializable.php
+++ b/src/SeriousJSON/JsonSerializable.php
@@ -89,7 +89,7 @@ abstract class JsonSerializable
      * @param bool $to_array Do we want the result as an array instead of a json encoded string?
      * @return string|array Returns a json encoded string or a key value array
      */
-    public function Serialize($nested = false, $to_array = false)
+    public function Serialize($nested = false, $to_array = false, IJsonUnflattener $callback = null)
     {
         // Initialize result set array
         $result = array();
@@ -106,18 +106,21 @@ abstract class JsonSerializable
             // Add var name and value to result set
             if (isset($value) && $value != null && $value instanceof JsonSerializable)
             {
-                if ($nested)
+                if ($nested) {
                     $result[$key] = $value->Serialize(true, true);
-                else if ($value instanceof IJsonIdentifiable && $value->flatIdentifier() !== null)
+                } else if ($value instanceof IJsonIdentifiable && $value->flatIdentifier() !== null) {
                     $result[$key] = $value->flatIdentifier();
-                else
-                    continue;
 
+                    if ($this instanceof IJsonIdentifiable && $callback != null)
+                        call_user_func([$callback, 'updateIndex'], $this, $value, false);
+                } else {
+                    continue;
+                } 
             }
             else if (isset($value) && $value != null && is_array($value)) {
                 $arrayType = self::getPropertyArrayType(new \ReflectionProperty($this, $key));
                 if ($arrayType != null)
-                    $result[$key] = self::SerializeArray($value, $nested, true);
+                    $result[$key] = self::SerializeArray(objects: $value, nested: $nested, to_array: true, callback: $callback);
                 else
                     $result[$key] = $value;
             }
@@ -139,7 +142,7 @@ abstract class JsonSerializable
      * @param bool $nested Indicates if we should also serialize nested object instances
      * @return string returns a json formatted string
      */
-    public static function SerializeArray($objects = [], $nested = false, $to_array = false)
+    public static function SerializeArray($objects = [], $nested = false, $to_array = false, IJsonUnflattener $callback = null)
     {
         // Initialize a an array of results
         $results = array();
@@ -149,7 +152,7 @@ abstract class JsonSerializable
             if ($object instanceof JsonSerializable)
             {
                 if ($nested)
-                    $results[] = $object->Serialize(nested: true, to_array: true);
+                    $results[] = $object->Serialize(nested: true, to_array: true, callback: $callback);
                 else if ($object instanceof IJsonIdentifiable && $object->flatIdentifier() !== null)
                     $results[] = $object->flatIdentifier();
                 else