diff --git a/DataStorage/Database/Query/Grammar/Grammar.php b/DataStorage/Database/Query/Grammar/Grammar.php
index 406420318..507d5dc76 100644
--- a/DataStorage/Database/Query/Grammar/Grammar.php
+++ b/DataStorage/Database/Query/Grammar/Grammar.php
@@ -245,6 +245,8 @@ class Grammar extends GrammarAbstract
             $expression .= '(' . $this->compileWhere($element['column'], $query->getPrefix()) . ')';
         }
 
+        // todo: handle IN(...) as operator
+
         if (isset($element['value'])) {
             $expression .= ' ' . strtoupper($element['operator']) . ' ' . $this->compileValue($element['value'], $query->getPrefix());
         }