diff --git a/h5p-metadata.class.php b/h5p-metadata.class.php
index 3a504e3..8462fa4 100644
--- a/h5p-metadata.class.php
+++ b/h5p-metadata.class.php
@@ -64,15 +64,16 @@ abstract class H5PMetadata {
       ',"authorComments":' . (isset($content->author_comments) ? json_encode($content->author_comments) : 'null') . '}';
   }
 
-
   /**
    * Make the metadata into an associative array keyed by the property names
    * @param mixed $metadata Array or object containing metadata
    * @param bool $include_title
+   * @param bool $include_missing For metadata fields not being set, skip 'em.
+   *                             Relevant for content upgrade
    * @param array $types
    * @return array
    */
-  public static function toDBArray($metadata, $include_title = true, &$types = array()) {
+  public static function toDBArray($metadata, $include_title = true, $include_missing = true, &$types = array()) {
     $fields = array();
 
     if (!is_array($metadata)) {
@@ -81,35 +82,43 @@ abstract class H5PMetadata {
 
     foreach (self::$fields as $key => $config) {
 
+      // Ignore title?
       if ($key === 'title' && !$include_title) {
         continue;
       }
 
-      if (array_key_exists($key, $metadata)) {
-        $value = $metadata[$key];
-        $db_field_name = strtolower(preg_replace('/(?<!^)[A-Z]/', '_$0', $key));
+      $exists = array_key_exists($key, $metadata);
 
-        switch ($config['type']) {
-          case 'text':
-            if ($value !== null && strlen($value) > $config['maxLength']) {
-              $value = mb_substr($value, 0, $config['maxLength']);
-            }
-            $types[] = '%s';
-            break;
-
-          case 'int':
-            $value = ($value !== null) ? intval($value): null;
-            $types[] = '%d';
-            break;
-
-          case 'json':
-            $value = ($value !== null) ? json_encode($value) : null;
-            $types[] = '%s';
-            break;
-        }
-
-        $fields[$db_field_name] = $value;
+      // Don't include missing fields
+      if (!$include_missing && !exists) {
+        continue;
       }
+
+      $value = $exists ? $metadata[$key] : null;
+
+      // lowerCamelCase to snake_case
+      $db_field_name = strtolower(preg_replace('/(?<!^)[A-Z]/', '_$0', $key));
+
+      switch ($config['type']) {
+        case 'text':
+          if ($value !== null && strlen($value) > $config['maxLength']) {
+            $value = mb_substr($value, 0, $config['maxLength']);
+          }
+          $types[] = '%s';
+          break;
+
+        case 'int':
+          $value = ($value !== null) ? intval($value) : null;
+          $types[] = '%d';
+          break;
+
+        case 'json':
+          $value = ($value !== null) ? json_encode($value) : null;
+          $types[] = '%s';
+          break;
+      }
+
+      $fields[$db_field_name] = $value;
     }
 
     return $fields;